diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index a913540f..8f7d3ef9 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -31,6 +31,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
@@ -61,6 +62,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
@@ -91,6 +93,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
@@ -121,6 +124,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         experimental: [false]
         include:
           - ruby-version: 'ruby-head'
@@ -149,7 +153,7 @@ jobs:
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '3.4'
+          ruby-version: '4.0'
           bundler-cache: true
           cache-version: 1
         continue-on-error: ${{ matrix.experimental }}
diff --git a/.rubocop.yml b/.rubocop.yml
index b169f1c8..0bb16826 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -13,62 +13,25 @@ AllCops:
     - 'gemfiles/*'
   TargetRubyVersion: 3.2
 
-Metrics/BlockLength:
-  Enabled: false
-
-Metrics/MethodLength:
+Gemspec/DevelopmentDependencies:
   Exclude:
-    - "lib/rubycritic/configuration.rb"
-
-Layout/LineLength:
-  Max: 120
-
-Style/Documentation:
-  Enabled: false
-
-Style/HashSyntax:
-  EnforcedShorthandSyntax: either
-
-Security/MarshalLoad:
-  Enabled: false
-  Include:
-    - 'lib/rubycritic/serializer.rb'
-
-Style/RedundantFreeze:
-  Enabled: false
-  Include:
-    - 'lib/rubycritic/core/smell.rb'
-    - 'lib/rubycritic/generators/json/simple.rb'
-    - 'lib/rubycritic/revision_comparator.rb'
-    - 'lib/rubycritic/source_control_systems/perforce.rb'
-    - 'lib/rubycritic/source_locator.rb'
-    - 'lib/rubycritic/version.rb'
+    - 'rubycritic.gemspec'
 
 Layout/BlockAlignment:
   Enabled: false
   Exclude:
     - 'features/step_definitions/rake_task_steps.rb'
 
-Naming/RescuedExceptionsVariableName:
-  Exclude:
-    - 'lib/rubycritic/analysers/coverage.rb'
-    - 'lib/rubycritic/cli/application.rb'
-    - 'lib/rubycritic/reporter.rb'
-
-Lint/EmptyClass:
-  Exclude:
-    - 'test/lib/rubycritic/reporter_test.rb'
+Layout/LineLength:
+  Max: 120
 
 Lint/ConstantDefinitionInBlock:
   Exclude:
     - 'test/lib/rubycritic/reporter_test.rb'
 
-Style/OpenStructUse:
+Lint/EmptyClass:
   Exclude:
-    - 'test/lib/rubycritic/generators/turbulence_test.rb'
-    - 'test/lib/rubycritic/generators/console_report_test.rb'
-    - 'test/lib/rubycritic/core/analysed_module_test.rb'
-    - 'test/analysers_test_helper.rb'
+    - 'test/lib/rubycritic/reporter_test.rb'
 
 Lint/MissingSuper:
   Exclude:
@@ -81,10 +44,6 @@ Lint/MissingSuper:
     - 'lib/rubycritic/generators/html/code_index.rb'
     - 'lib/rubycritic/generators/html/line.rb'
 
-Gemspec/DevelopmentDependencies:
-  Exclude:
-    - 'rubycritic.gemspec'
-
 Lint/StructNewOverride:
   Exclude:
     - 'lib/rubycritic/source_control_systems/git/churn.rb'
@@ -93,10 +52,47 @@ Metrics/AbcSize:
   Exclude:
     - 'lib/rubycritic/configuration.rb'
 
+Metrics/BlockLength:
+  Enabled: false
+
 Metrics/MethodLength:
   Exclude:
     - 'lib/rubycritic/configuration.rb'
 
+Naming/RescuedExceptionsVariableName:
+  Exclude:
+    - 'lib/rubycritic/analysers/coverage.rb'
+    - 'lib/rubycritic/cli/application.rb'
+    - 'lib/rubycritic/reporter.rb'
+
+Security/MarshalLoad:
+  Enabled: false
+  Include:
+    - 'lib/rubycritic/serializer.rb'
+
+Style/Documentation:
+  Enabled: false
+
+Style/HashSyntax:
+  EnforcedShorthandSyntax: either
+
+Style/OpenStructUse:
+  Exclude:
+    - 'test/lib/rubycritic/generators/turbulence_test.rb'
+    - 'test/lib/rubycritic/generators/console_report_test.rb'
+    - 'test/lib/rubycritic/core/analysed_module_test.rb'
+    - 'test/analysers_test_helper.rb'
+
 Style/RedundantAssignment:
   Exclude:
     - 'lib/rubycritic/configuration.rb'
+
+Style/RedundantFreeze:
+  Enabled: false
+  Include:
+    - 'lib/rubycritic/core/smell.rb'
+    - 'lib/rubycritic/generators/json/simple.rb'
+    - 'lib/rubycritic/revision_comparator.rb'
+    - 'lib/rubycritic/source_control_systems/perforce.rb'
+    - 'lib/rubycritic/source_locator.rb'
+    - 'lib/rubycritic/version.rb'
diff --git a/CHANGELOG.md b/CHANGELOG.md
index efd38d62..165a8406 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,12 @@
 * [CHORE] ...
 * [FEATURE] ...
 
+* [CHORE] Bump fakefs (by [@faisal][])
+* [CHORE] In RubyCritic::Generator::Html::Line, require cgi/escape rather than cgi, as the latter is removed from Ruby 4.0 (by [@faisal][])
+* [CHORE] In RubyCritic::Generator::Html::Line, require cgi/escape rather than cgi, as the latter is removed from Ruby 4.0 (by [@faisal][])
+* [CHORE] Run Markdown linting in CI using Ruby 4.0 (by [@faisal][])
+* [CHORE] Start testing library with Ruby 4.0 (by [@etagwerker][])
+* [CHORE] Sorted cops in .rubocop.yml to make it easier for maintainers to add or modify cop settings, and unified duplicate Metrics/MethodLength settings. (by [@faisal][])
 * [CHANGE] Drop support for Ruby 3.1.x and JRuby 9.4 (by [@faisal][])
 * [CHANGE] Add CI support for JRuby 10.0 (by [@faisal][])
 * [CHANGE] Bump cucumber dependency (by [@faisal][])
diff --git a/lib/rubycritic/analysers/smells/reek.rb b/lib/rubycritic/analysers/smells/reek.rb
index db5cf0cf..cfe8ee5e 100644
--- a/lib/rubycritic/analysers/smells/reek.rb
+++ b/lib/rubycritic/analysers/smells/reek.rb
@@ -10,7 +10,10 @@ class ReekSmells
       include Colorize
 
       def initialize(analysed_modules)
-        @analysed_modules = analysed_modules.reject { Reek.configuration.path_excluded?(_1.pathname) }
+        @analysed_modules = analysed_modules.reject do |mod|
+          path_string = mod.pathname.to_s
+          Reek.configuration.path_excluded?(Pathname.new(path_string))
+        end
       end
 
       def run
diff --git a/lib/rubycritic/generators/html/line.rb b/lib/rubycritic/generators/html/line.rb
index c5e3cd15..51fc5e6a 100644
--- a/lib/rubycritic/generators/html/line.rb
+++ b/lib/rubycritic/generators/html/line.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'cgi'
+require 'cgi/escape'
 require 'rubycritic/generators/html/base'
 
 module RubyCritic
diff --git a/rubycritic.gemspec b/rubycritic.gemspec
index ff4adb93..afadd7a3 100644
--- a/rubycritic.gemspec
+++ b/rubycritic.gemspec
@@ -54,8 +54,7 @@ Gem::Specification.new do |spec|
   end
   spec.add_development_dependency 'cucumber', '~> 10.2.0', '>= 10.1.0'
   spec.add_development_dependency 'diff-lcs', '~> 1.3'
-  spec.add_development_dependency 'fakefs', '~> 3.0.0'
-  spec.add_development_dependency 'irb'
+  spec.add_development_dependency 'fakefs', '~> 3.2.0'
   spec.add_development_dependency 'mdl', '~> 0.15.0', '>= 0.12.0'
   spec.add_development_dependency 'minitest', '~> 6.0.0'
   spec.add_development_dependency 'minitest-around', '~> 0.6.0'
diff --git a/test/fakefs_helper.rb b/test/fakefs_helper.rb
index a63f7abc..282c3b88 100644
--- a/test/fakefs_helper.rb
+++ b/test/fakefs_helper.rb
@@ -24,3 +24,14 @@ def home(user = Etc.getlogin)
   end
 end
 FakeFS::Dir.singleton_class.prepend(FakeFSPatch)
+
+# Patch FakeFS::Pathname to include the path method for Ruby 4.0.0 compatibility
+# This is needed because Reek's configuration internally calls Pathname#== which
+# requires the path method to be present
+module FakeFS
+  class Pathname
+    def path
+      to_s
+    end
+  end
+end