diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ecac63ae2..ce6c419db 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,7 +11,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
         # 3.0 is interpreted as 3
-        ruby: ["3.0", 3.1, 3.2, 3.3, 3.4]
+        ruby: ["3.0", 3.1, 3.2, 3.3, 3.4, "4.0"]
         include:
           - os: ubuntu-latest
             container: ruby:2.2
diff --git a/Gemfile b/Gemfile
index 1048c3434..771068f7e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,6 +2,12 @@
 source 'https://rubygems.org'
 
 group :development do
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('4.0.0')
+    gem 'benchmark'
+    gem 'irb'
+    gem 'logger'
+    gem 'ostruct'
+  end
   gem 'rspec', '>= 3.11.0'
   gem 'rake'
   gem 'rdoc', '= 6.1.2.1'
diff --git a/lib/yard/rubygems/specification.rb b/lib/yard/rubygems/specification.rb
index 905ba067a..1701ce046 100644
--- a/lib/yard/rubygems/specification.rb
+++ b/lib/yard/rubygems/specification.rb
@@ -11,7 +11,7 @@ def has_yardoc
     @has_rdoc == 'yard'
   end
 
-  undef has_rdoc?
+  undef has_rdoc? if method_defined?(:has_rdoc?)
   def has_rdoc?
     (@has_rdoc ||= true) && @has_rdoc != 'yard'
   end
diff --git a/spec/handlers/ruby/base_spec.rb b/spec/handlers/ruby/base_spec.rb
index 3121bf801..20871281f 100644
--- a/spec/handlers/ruby/base_spec.rb
+++ b/spec/handlers/ruby/base_spec.rb
@@ -37,7 +37,7 @@ class StringHandler < Handlers::Ruby::Base
       handles "x"
     end
     allow(Handlers::Base).to receive(:subclasses).and_return [StringHandler]
-    ast = Parser::Ruby::RubyParser.parse("if x == 2 then true end").ast
+    ast = YARD::Parser::Ruby::RubyParser.parse("if x == 2 then true end").ast
     valid StringHandler, ast[0][0][0]
     invalid StringHandler, ast[0][1]
   end
@@ -56,7 +56,7 @@ class RegexHandler < Handlers::Ruby::Base
       handles(/^if x ==/)
     end
     allow(Handlers::Base).to receive(:subclasses).and_return [RegexHandler]
-    ast = Parser::Ruby::RubyParser.parse("if x == 2 then true end").ast
+    ast = YARD::Parser::Ruby::RubyParser.parse("if x == 2 then true end").ast
     valid RegexHandler, ast
     invalid RegexHandler, ast[0][1]
   end
@@ -75,7 +75,7 @@ class MethCallHandler < Handlers::Ruby::Base
       handles method_call(:meth)
     end
     allow(Handlers::Base).to receive(:subclasses).and_return [MethCallHandler]
-    ast = Parser::Ruby::RubyParser.parse(<<-"eof").ast
+    ast = YARD::Parser::Ruby::RubyParser.parse(<<-"eof").ast
       meth                   # 0
       meth()                 # 1
       meth(1,2,3)            # 2
diff --git a/yard.gemspec b/yard.gemspec
index fa43eff9b..81831c81b 100644
--- a/yard.gemspec
+++ b/yard.gemspec
@@ -20,4 +20,8 @@ Gem::Specification.new do |s|
   s.executables   = ['yard', 'yardoc', 'yri']
   s.license = 'MIT' if s.respond_to?(:license=)
   s.metadata['yard.run'] = 'yri'
+  if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('4.0.0')
+    s.add_dependency 'irb'
+    s.add_dependency 'logger'
+  end
 end