diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 6e4eee24..aec4d979 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -18,6 +18,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby:
+          - '4.0'
           - '3.4'
           - '3.3'
           - '3.2'
diff --git a/rack-attack.gemspec b/rack-attack.gemspec
index 4d03a71b..f9d2750a 100644
--- a/rack-attack.gemspec
+++ b/rack-attack.gemspec
@@ -29,7 +29,6 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'rack', ">= 1.0", "< 4"
 
   s.add_development_dependency 'appraisal', '~> 2.2'
-  s.add_development_dependency "bundler", ">= 1.17", "< 3.0"
   s.add_development_dependency 'minitest', "~> 5.11"
   s.add_development_dependency "minitest-stub-const", "~> 0.6"
   s.add_development_dependency 'rack-test', "~> 2.0"
diff --git a/spec/acceptance/cache_store_config_with_rails_spec.rb b/spec/acceptance/cache_store_config_with_rails_spec.rb
index 66bb76a8..25b68975 100644
--- a/spec/acceptance/cache_store_config_with_rails_spec.rb
+++ b/spec/acceptance/cache_store_config_with_rails_spec.rb
@@ -2,7 +2,6 @@
 
 require_relative "../spec_helper"
 require "minitest/stub_const"
-require "ostruct"
 
 describe "Cache store config with Rails" do
   before do
@@ -13,7 +12,8 @@
 
   unless defined?(Rails)
     it "fails when Rails.cache is not set" do
-      Object.stub_const(:Rails, OpenStruct.new(cache: nil)) do
+      rails = Struct.new(:cache).new(nil)
+      Object.stub_const(:Rails, rails) do
         assert_raises(Rack::Attack::MissingStoreError) do
           get "/", {}, "REMOTE_ADDR" => "1.2.3.4"
         end
@@ -22,7 +22,8 @@
   end
 
   it "works when Rails.cache is set" do
-    Object.stub_const(:Rails, OpenStruct.new(cache: ActiveSupport::Cache::MemoryStore.new)) do
+    rails = Struct.new(:cache).new(ActiveSupport::Cache::MemoryStore.new)
+    Object.stub_const(:Rails, rails) do
       get "/", {}, "REMOTE_ADDR" => "1.2.3.4"
 
       assert_equal 200, last_response.status