diff --git a/.tool-versions b/.tool-versions
index 58766197c..efdfc37ef 100644
--- a/.tool-versions
+++ b/.tool-versions
@@ -1 +1 @@
-ruby 3.4.8
+ruby 4.0.4
diff --git a/AGENTS.md b/AGENTS.md
index 255e5dbf7..b53622f7a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -34,7 +34,7 @@ docker compose up
 - Full suite: `docker compose run --rm api rspec`
 - Single spec: `docker compose run --rm api rspec spec/path/to/spec.rb`
 - Lint: `docker compose run --rm api bundle exec rubocop`
-- CI: GitHub Actions with Ruby 3.4, Postgres 12, Redis.
+- CI: GitHub Actions with Ruby 4, Postgres 12, Redis.
 
 ## Where to Look First
 - Routes: `config/routes.rb`. Auth: `config/initializers/omniauth.rb`, `app/helpers/authentication_helper.rb`, `app/controllers/concerns/identifiable.rb`.
diff --git a/Dockerfile b/Dockerfile
index 2275fb630..c16ae8035 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM ruby:3.4.8-slim-trixie AS base
+FROM ruby:4.0.4-slim-trixie AS base
 RUN gem install bundler \
   && apt-get update \
   && apt-get upgrade --yes \
diff --git a/Gemfile b/Gemfile
index 9e57e8efc..68787a2e3 100644
--- a/Gemfile
+++ b/Gemfile
@@ -9,7 +9,7 @@ gem 'administrate', '~> 1.0.0'
 gem 'administrate-field-active_storage'
 gem 'aws-sdk-s3', require: false
 gem 'base64', '~> 0.3'
-gem 'bootsnap', require: false
+gem 'bootsnap', '>= 1.24.4', require: false
 gem 'cancancan', '~> 3.3'
 gem 'countries'
 gem 'csv', '~> 3.3'
diff --git a/Gemfile.lock b/Gemfile.lock
index 51c4d7d4a..a2f4553d6 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -118,7 +118,7 @@ GEM
       aws-eventstream (~> 1, >= 1.0.2)
     base64 (0.3.0)
     bigdecimal (4.1.2)
-    bootsnap (1.24.1)
+    bootsnap (1.24.4)
       msgpack (~> 1.2)
     builder (3.3.0)
     bullet (8.1.1)
@@ -183,6 +183,7 @@ GEM
       logger
     faraday-net_http (3.4.2)
       net-http (~> 0.5)
+    ffi (1.17.2)
     ffi (1.17.2-aarch64-linux-gnu)
     ffi (1.17.2-arm64-darwin)
     ffi (1.17.2-x86_64-linux-gnu)
@@ -579,7 +580,7 @@ DEPENDENCIES
   awesome_print
   aws-sdk-s3
   base64 (~> 0.3)
-  bootsnap
+  bootsnap (>= 1.24.4)
   bullet
   cancancan (~> 3.3)
   capybara
@@ -643,7 +644,7 @@ DEPENDENCIES
   webmock
 
 RUBY VERSION
-  ruby 3.4.8
+  ruby 4.0.4
 
 BUNDLED WITH
   4.0.10