diff --git a/.devcontainer/post-create.sh b/.devcontainer/post-create.sh
index 723347c6cc..35faa0b7d5 100755
--- a/.devcontainer/post-create.sh
+++ b/.devcontainer/post-create.sh
@@ -3,9 +3,9 @@ RUBY_VERSION="$(cat .ruby-version | tr -d '\n')"
 # copy the file only if it doesn't already exist
 cp -n .devcontainer/.env.codespaces .env
 
-# If the project's required ruby version changes from 3.3.10, this command
+# If the project's required ruby version changes from 4.0.2, this command
 # will download and compile the correct version, but it will take a long time.
-if [ "$RUBY_VERSION" != "3.3.10" ]; then
+if [ "$RUBY_VERSION" != "4.0.2" ]; then
 	rvm install $RUBY_VERSION
 	rvm use $RUBY_VERSION
 	echo "Ruby $RUBY_VERSION installed"
diff --git a/.ruby-version b/.ruby-version
index 5f6fc5edc2..4d54daddb6 100644
--- a/.ruby-version
+++ b/.ruby-version
@@ -1 +1 @@
-3.3.10
+4.0.2
diff --git a/.tool-versions b/.tool-versions
index c8799e1dc1..2b58a4f8e5 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -1,2 +1,2 @@
-ruby 3.3.10
+ruby 4.0.2
 nodejs 24.13.0
diff --git a/Dockerfile b/Dockerfile
index 228419622c..559a4cebbd 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -4,7 +4,7 @@ ARG ROOT=/usr/src/app/
 
 FROM node:24-alpine AS node-source
 
-FROM ruby:3.3.10-alpine AS build
+FROM ruby:4.0.2-alpine AS build
   ARG ROOT
   WORKDIR $ROOT
 
@@ -20,7 +20,7 @@ FROM ruby:3.3.10-alpine AS build
   COPY Gemfile* $ROOT
   RUN bundle install
 
-FROM ruby:3.3.10-alpine
+FROM ruby:4.0.2-alpine
   ARG ROOT
   WORKDIR $ROOT
 
diff --git a/Gemfile b/Gemfile
index 0a02501b3e..e75efcc7af 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,7 +2,7 @@
 
 source "https://rubygems.org"
 
-ruby "3.3.10"
+ruby "4.0.2"
 gem "rails", "~> 7.2"
 
 gem "after_party" # Post-deployment tasks
diff --git a/Gemfile.lock b/Gemfile.lock
index e09dbd501b..9dca551e17 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -236,7 +236,7 @@ GEM
     faraday-net_http_persistent (1.2.0)
     faraday-patron (1.0.0)
     faraday-rack (1.0.0)
-    faraday-retry (1.0.3)
+    faraday-retry (1.0.4)
     faraday_middleware (1.2.1)
       faraday (~> 1.0)
     ffi (1.17.2)
@@ -802,7 +802,7 @@ DEPENDENCIES
   wicked
 
 RUBY VERSION
-   ruby 3.3.10p183
+  ruby 4.0.2
 
 BUNDLED WITH
-   2.6.6
+  4.0.9
diff --git a/README.md b/README.md
index c1ef6205dc..14c66e556a 100644
--- a/README.md
+++ b/README.md
@@ -102,7 +102,7 @@ The complete [role description of a CASA volunteer](https://pgcasa.org/volunteer
 
 **Ruby**
 1. Install a ruby version manager: [rvm](https://rvm.io/) or [rbenv](https://github.com/rbenv/rbenv)
-1. when you cd into the project directory, let your version manager install the ruby version in `.ruby-version`. Right now that's Ruby 3.3.10
+1. when you cd into the project directory, let your version manager install the ruby version in `.ruby-version`. Right now that's Ruby 4.0.2
 1. `gem install bundler`
 
 **node.js**
diff --git a/app/services/case_contacts_contact_dates.rb b/app/services/case_contacts_contact_dates.rb
index d9304f92dd..372fc68110 100644
--- a/app/services/case_contacts_contact_dates.rb
+++ b/app/services/case_contacts_contact_dates.rb
@@ -34,6 +34,6 @@ def chron_sort(case_contacts)
   end
 
   def order_and_format(case_contacts)
-    case_contacts.then { chron_sort _1 }.then { format_dates _1 }
+    case_contacts.then { chron_sort it }.then { format_dates it }
   end
 end
diff --git a/doc/LINUX_SETUP.md b/doc/LINUX_SETUP.md
index fd7c1e3ac8..f34ca0df2c 100644
--- a/doc/LINUX_SETUP.md
+++ b/doc/LINUX_SETUP.md
@@ -84,7 +84,7 @@ git clone https://github.com/rbenv/rbenv.git ~/.rbenv
 #   fetch list of ruby versions
 git clone https://github.com/rbenv/ruby-build.git "$(rbenv root)"/plugins/ruby-build
 
-rbenv install 3.3.10
+rbenv install 4.0.2
 ```
 
 If you would like RVM instead of rbenv
@@ -95,9 +95,9 @@ gpg --keyserver hkp://pool.sks-keyservers.net --recv-keys 409B6B1796C275462A1703
 \curl -sSL https://get.rvm.io | bash
 . ./.bashrc
 rvm get head
-rvm install 3.3.10
-rvm alias create ruby 3.3.10
-rvm alias create default ruby-3.3.10
+rvm install 4.0.2
+rvm alias create ruby 4.0.2
+rvm alias create default ruby-4.0.2
 ```
 
 ```# Download the Chrome browser (for RSpec testing):
diff --git a/doc/MAC_SETUP.md b/doc/MAC_SETUP.md
index 20007e0a19..2cd5df298e 100644
--- a/doc/MAC_SETUP.md
+++ b/doc/MAC_SETUP.md
@@ -56,17 +56,17 @@ Next, install the version of Ruby that this project uses. This can be found by c
 To install the appropriate ruby version, run:
 
 ```bash
-rbenv install 3.3.10
+rbenv install 4.0.2
 ```
 
-(Do not forget to switch 3.3.10 to the appropriate version)
+(Do not forget to switch 4.0.2 to the appropriate version)
 
 Finally, run:
 
 ```bash
-rbenv local 3.3.10
+rbenv local 4.0.2
 ```
-(Do not forget to swtich 3.3.10 to the appropriate version)
+(Do not forget to switch 4.0.2 to the appropriate version)
 
 ## Nodejs
 
diff --git a/doc/WSL_SETUP.md b/doc/WSL_SETUP.md
index e6e0436f09..08000015f9 100644
--- a/doc/WSL_SETUP.md
+++ b/doc/WSL_SETUP.md
@@ -28,7 +28,7 @@ You will need the following local tools installed:
 
 Install a ruby version manager like [rbenv](https://github.com/rbenv/rbenv#installation)
 
-  **Be sure to install the ruby version in `.ruby-version`. Right now that's Ruby 3.3.10.**
+  **Be sure to install the ruby version in `.ruby-version`. Right now that's Ruby 4.0.2.**
 
 Instructions for rbenv:
 
@@ -48,13 +48,13 @@ Instructions for rbenv:
 
 5.  **[Install Ruby](https://github.com/rbenv/rbenv#installing-ruby-versions)**
 
-      **Be sure to install the ruby version in `.ruby-version`. Right now that's Ruby 3.3.10.**
+      **Be sure to install the ruby version in `.ruby-version`. Right now that's Ruby 4.0.2.**
 
-      `rbenv install 3.3.10`
+      `rbenv install 4.0.2`
 
 6. **Set a Ruby version to finish installation and start**
 
-    `rbenv global 3.3.10` OR `rbenv local 3.3.10`
+    `rbenv global 4.0.2` OR `rbenv local 4.0.2`
 
 #### Troubleshooting
     If you are on Ubuntu in Windows Subsystem for Linux (WSL) and `rbenv install` indicates that the Ruby version is unavailable, you might be using Ubuntu's default install of `ruby-build`, which only comes with old installs of Ruby (ending before 2.6.) You should uninstall rvm and ruby-build's apt packages (`apt remove rvm ruby-build`) and install them with Git like this:
diff --git a/flake.nix b/flake.nix
index 79a34b4c41..06afa4a884 100644
--- a/flake.nix
+++ b/flake.nix
@@ -39,7 +39,7 @@
 
         gemConfig = { };
         # See available versions here: https://github.com/bobvanderlinden/nixpkgs-ruby/blob/master/ruby/versions.json
-        ruby = pkgs."ruby-3.3.10";
+        ruby = pkgs."ruby-4.0.2";
 
         bundixcli = bundix.packages.${system}.default;
       in rec {