Created a Add_4_Sum_Problem.java

src/test/java/com/thealgorithms/misc/Add_4_Sum_Problem.java

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+import java.util.*;
+
+public class FourSum {
+    public static List<List<Integer>> fourSum(int[] arr, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        if (arr == null || arr.length < 4) return result;
+
+        Arrays.sort(arr);
+        int n = arr.length;
+
+
+        for (int i = 0; i < n - 3; i++) {
+            if (i > 0 && arr[i] == arr[i - 1]) continue;  // Skip duplicates.
+
+            for (int j = i + 1; j < n - 2; j++) {
+                if (j > i + 1 && arr[j] == arr[j - 1]) continue;  // Skip duplicates.
+
+                int left = j + 1;
+                int right = n - 1;
+
+                while (left < right) {
+                    int sum = arr[i] + arr[j] + arr[left] + arr[right];
+
+                    if (sum == target) {
+                        result.add(Arrays.asList(arr[i], arr[j], arr[left], arr[right]));
+
+
+                        while (left < right && arr[left] == arr[left + 1]) left++;
+                        while (left < right && arr[right] == arr[right - 1]) right--;
+
+                        left++;
+                        right--;
+                    } else if (sum < target) {
+                        left++;  
+                    } else {
+                        right--; 
+                    }
+                }
+            }
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        Scanner scanner = new Scanner(System.in);
+
+        System.out.print("Enter the number of elements in the array: ");
+        int n = scanner.nextInt();
+        int[] arr = new int[n];
+
+        System.out.println("Enter the elements of the array:");
+        for (int i = 0; i < n; i++) {
+            arr[i] = scanner.nextInt();
+        }
+
+        System.out.print("Enter the target value: ");
+        int target = scanner.nextInt();
+
+        List<List<Integer>> result = fourSum(arr, target);
+        System.out.println("Unique quadruplets that sum to " + target + ":");
+        for (List<Integer> quad : result) {
+            System.out.println(quad);
+        }
+
+        scanner.close();
+    }
+}