diff --git a/KNOWN-ISSUES.md b/KNOWN-ISSUES.md
index 65d00bb2b..9e976bb68 100644
--- a/KNOWN-ISSUES.md
+++ b/KNOWN-ISSUES.md
@@ -86,21 +86,7 @@ km search 'content:"user:password"'
 
 ## Configuration Limitations
 
-### 4. Index ID Hardcoded
-
-**Status:** Known limitation
-
-**Issue:** Index ID is hardcoded as `"fts-main"` instead of being loaded from configuration.
-
-**Location:** `src/Core/Search/NodeSearchService.cs:77`
-
-**Impact:** Cannot configure multiple indexes per node.
-
-**Fix Required:** Load index configuration from node settings.
-
----
-
-### 5. Index Weights Not Configurable
+### 4. Index Weights Not Configurable
 
 **Status:** Known limitation
 
diff --git a/src/Core/Search/NodeSearchService.cs b/src/Core/Search/NodeSearchService.cs
index 106fbb344..b2883d5ab 100644
--- a/src/Core/Search/NodeSearchService.cs
+++ b/src/Core/Search/NodeSearchService.cs
@@ -14,6 +14,7 @@ namespace KernelMemory.Core.Search;
 public sealed class NodeSearchService
 {
     private readonly string _nodeId;
+    private readonly string _indexId;
     private readonly IFtsIndex _ftsIndex;
     private readonly IContentStorage _contentStorage;
 
@@ -23,9 +24,15 @@ public sealed class NodeSearchService
     /// <param name="nodeId">The node ID this service operates on.</param>
     /// <param name="ftsIndex">The FTS index for this node.</param>
     /// <param name="contentStorage">The content storage for loading full records.</param>
-    public NodeSearchService(string nodeId, IFtsIndex ftsIndex, IContentStorage contentStorage)
+    /// <param name="indexId">Optional index ID for this FTS index. Defaults to SearchConstants.DefaultFtsIndexId.</param>
+    public NodeSearchService(
+        string nodeId,
+        IFtsIndex ftsIndex,
+        IContentStorage contentStorage,
+        string indexId = SearchConstants.DefaultFtsIndexId)
     {
         this._nodeId = nodeId;
+        this._indexId = indexId;
         this._ftsIndex = ftsIndex;
         this._contentStorage = contentStorage;
     }
@@ -74,7 +81,7 @@ public NodeSearchService(string nodeId, IFtsIndex ftsIndex, IContentStorage cont
                     {
                         RecordId = content.Id,
                         NodeId = this._nodeId,
-                        IndexId = "fts-main", // TODO: Get from index config
+                        IndexId = this._indexId,
                         ChunkId = null,
                         BaseRelevance = (float)match.Score,
                         Title = content.Title,
diff --git a/src/Core/Search/SearchConstants.cs b/src/Core/Search/SearchConstants.cs
index 90bc229c7..f6e3b2812 100644
--- a/src/Core/Search/SearchConstants.cs
+++ b/src/Core/Search/SearchConstants.cs
@@ -118,4 +118,10 @@ public static class SearchConstants
     /// Minimum relevance score.
     /// </summary>
     public const float MinRelevanceScore = 0.0f;
+
+    /// <summary>
+    /// Default FTS index ID used when not specified in configuration.
+    /// This is the identifier assigned to search results from the full-text search index.
+    /// </summary>
+    public const string DefaultFtsIndexId = "fts-main";
 }
diff --git a/tests/Core.Tests/Search/NodeSearchServiceIndexIdTests.cs b/tests/Core.Tests/Search/NodeSearchServiceIndexIdTests.cs
new file mode 100644
index 000000000..628326182
--- /dev/null
+++ b/tests/Core.Tests/Search/NodeSearchServiceIndexIdTests.cs
@@ -0,0 +1,233 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using KernelMemory.Core.Search;
+using KernelMemory.Core.Search.Models;
+using KernelMemory.Core.Search.Query.Ast;
+using KernelMemory.Core.Storage;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Moq;
+
+namespace KernelMemory.Core.Tests.Search;
+
+/// <summary>
+/// Unit tests for NodeSearchService index ID configuration.
+/// Tests that index ID is properly configurable and not hardcoded.
+/// Issue: Known Issue #4 - Index ID was hardcoded as "fts-main".
+/// </summary>
+public sealed class NodeSearchServiceIndexIdTests : IDisposable
+{
+    private readonly string _tempDir;
+    private readonly Mock<ILogger<SqliteFtsIndex>> _mockFtsLogger;
+    private readonly Mock<ILogger<ContentStorageService>> _mockStorageLogger;
+    private readonly List<ContentStorageDbContext> _contexts = [];
+    private readonly List<SqliteFtsIndex> _ftsIndexes = [];
+
+    public NodeSearchServiceIndexIdTests()
+    {
+        this._tempDir = Path.Combine(Path.GetTempPath(), $"km-index-id-test-{Guid.NewGuid():N}");
+        Directory.CreateDirectory(this._tempDir);
+        this._mockFtsLogger = new Mock<ILogger<SqliteFtsIndex>>();
+        this._mockStorageLogger = new Mock<ILogger<ContentStorageService>>();
+    }
+
+    public void Dispose()
+    {
+        // Dispose all tracked resources
+        foreach (var ftsIndex in this._ftsIndexes)
+        {
+            ftsIndex.Dispose();
+        }
+
+        foreach (var context in this._contexts)
+        {
+            context.Dispose();
+        }
+
+        try
+        {
+            if (Directory.Exists(this._tempDir))
+            {
+                Directory.Delete(this._tempDir, true);
+            }
+        }
+        catch (IOException)
+        {
+            // Ignore cleanup errors
+        }
+    }
+
+    /// <summary>
+    /// Tests that NodeSearchService uses the provided index ID in search results.
+    /// The index ID should be configurable, not hardcoded.
+    /// </summary>
+    [Fact]
+    public async Task SearchAsync_WithCustomIndexId_ResultsContainThatIndexId()
+    {
+        // Arrange
+        const string customIndexId = "custom-fts-index";
+        const string nodeId = "test-node";
+
+        var (ftsIndex, storage) = this.CreateIndexAndStorage("custom_index");
+
+        // Insert content
+        await storage.UpsertAsync(new KernelMemory.Core.Storage.Models.UpsertRequest
+        {
+            Content = "Test content for custom index ID verification",
+            MimeType = "text/plain"
+        }, CancellationToken.None).ConfigureAwait(false);
+
+        // Create NodeSearchService with custom index ID
+        var nodeService = new NodeSearchService(nodeId, ftsIndex, storage, customIndexId);
+
+        var request = new SearchRequest
+        {
+            Query = "content",
+            Limit = 10,
+            MinRelevance = 0.0f
+        };
+
+        // Act - SearchIndexResult is returned by NodeSearchService.SearchAsync
+        var queryNode = new TextSearchNode { SearchText = "content" };
+        var (results, _) = await nodeService.SearchAsync(
+            queryNode,
+            request,
+            CancellationToken.None).ConfigureAwait(false);
+
+        // Assert - SearchIndexResult has IndexId property
+        Assert.NotEmpty(results);
+        Assert.All(results, r => Assert.Equal(customIndexId, r.IndexId));
+    }
+
+    /// <summary>
+    /// Tests that the default index ID constant is used when not specified.
+    /// Ensures backward compatibility.
+    /// </summary>
+    [Fact]
+    public async Task SearchAsync_WithDefaultIndexId_UsesSearchConstantsDefault()
+    {
+        // Arrange
+        const string nodeId = "test-node";
+
+        var (ftsIndex, storage) = this.CreateIndexAndStorage("default_index");
+
+        // Insert content
+        await storage.UpsertAsync(new KernelMemory.Core.Storage.Models.UpsertRequest
+        {
+            Content = "Test content for default index ID verification",
+            MimeType = "text/plain"
+        }, CancellationToken.None).ConfigureAwait(false);
+
+        // Create NodeSearchService WITHOUT specifying index ID (uses default)
+        var nodeService = new NodeSearchService(nodeId, ftsIndex, storage);
+
+        var request = new SearchRequest
+        {
+            Query = "content",
+            Limit = 10,
+            MinRelevance = 0.0f
+        };
+
+        // Act
+        var queryNode = new TextSearchNode { SearchText = "content" };
+        var (results, _) = await nodeService.SearchAsync(
+            queryNode,
+            request,
+            CancellationToken.None).ConfigureAwait(false);
+
+        // Assert
+        Assert.NotEmpty(results);
+        Assert.All(results, r => Assert.Equal(SearchConstants.DefaultFtsIndexId, r.IndexId));
+    }
+
+    /// <summary>
+    /// Tests that different nodes can have different index IDs.
+    /// Validates multi-node, multi-index scenarios.
+    /// </summary>
+    [Fact]
+    public async Task SearchAsync_MultipleNodesWithDifferentIndexIds_EachHasCorrectIndexId()
+    {
+        // Arrange
+        const string node1Id = "node1";
+        const string node1IndexId = "node1-fts-index";
+        const string node2Id = "node2";
+        const string node2IndexId = "node2-fts-index";
+
+        var (ftsIndex1, storage1) = this.CreateIndexAndStorage("node1_db");
+        var (ftsIndex2, storage2) = this.CreateIndexAndStorage("node2_db");
+
+        // Insert content into both nodes
+        await storage1.UpsertAsync(new KernelMemory.Core.Storage.Models.UpsertRequest
+        {
+            Content = "Content in first node with custom index",
+            MimeType = "text/plain"
+        }, CancellationToken.None).ConfigureAwait(false);
+
+        await storage2.UpsertAsync(new KernelMemory.Core.Storage.Models.UpsertRequest
+        {
+            Content = "Content in second node with different index",
+            MimeType = "text/plain"
+        }, CancellationToken.None).ConfigureAwait(false);
+
+        // Create NodeSearchServices with different index IDs
+        var node1Service = new NodeSearchService(node1Id, ftsIndex1, storage1, node1IndexId);
+        var node2Service = new NodeSearchService(node2Id, ftsIndex2, storage2, node2IndexId);
+
+        var queryNode = new TextSearchNode { SearchText = "content" };
+        var request = new SearchRequest
+        {
+            Query = "content",
+            Limit = 10,
+            MinRelevance = 0.0f
+        };
+
+        // Act - Test each node service directly
+        var (results1, _) = await node1Service.SearchAsync(queryNode, request, CancellationToken.None).ConfigureAwait(false);
+        var (results2, _) = await node2Service.SearchAsync(queryNode, request, CancellationToken.None).ConfigureAwait(false);
+
+        // Assert - Each node returns results with its own index ID
+        Assert.NotEmpty(results1);
+        Assert.NotEmpty(results2);
+        Assert.All(results1, r => Assert.Equal(node1IndexId, r.IndexId));
+        Assert.All(results2, r => Assert.Equal(node2IndexId, r.IndexId));
+    }
+
+    /// <summary>
+    /// Tests that SearchConstants.DefaultFtsIndexId constant has the expected value.
+    /// Validates the constant is properly defined.
+    /// </summary>
+    [Fact]
+    public void DefaultFtsIndexId_HasExpectedValue()
+    {
+        // Assert
+        Assert.Equal("fts-main", SearchConstants.DefaultFtsIndexId);
+    }
+
+    /// <summary>
+    /// Helper method to create FTS index and storage for testing.
+    /// Tracks created resources for proper disposal.
+    /// </summary>
+    /// <param name="dbPrefix">Prefix for database file names to ensure uniqueness.</param>
+    /// <returns>Tuple of FTS index and storage service.</returns>
+    private (SqliteFtsIndex ftsIndex, ContentStorageService storage) CreateIndexAndStorage(string dbPrefix)
+    {
+        var ftsDbPath = Path.Combine(this._tempDir, $"{dbPrefix}_fts.db");
+        var contentDbPath = Path.Combine(this._tempDir, $"{dbPrefix}_content.db");
+
+        var ftsIndex = new SqliteFtsIndex(ftsDbPath, enableStemming: true, this._mockFtsLogger.Object);
+        this._ftsIndexes.Add(ftsIndex);
+
+        var options = new DbContextOptionsBuilder<ContentStorageDbContext>()
+            .UseSqlite($"Data Source={contentDbPath}")
+            .Options;
+        var context = new ContentStorageDbContext(options);
+        this._contexts.Add(context);
+        context.Database.EnsureCreated();
+
+        var cuidGenerator = new CuidGenerator();
+        var searchIndexes = new Dictionary<string, ISearchIndex> { ["fts"] = ftsIndex };
+        var storage = new ContentStorageService(context, cuidGenerator, this._mockStorageLogger.Object, searchIndexes);
+
+        return (ftsIndex, storage);
+    }
+}