diff --git a/README.md b/README.md
index 017ca5e..57bd31d 100644
--- a/README.md
+++ b/README.md
@@ -51,8 +51,8 @@ Install the PHP dependencies, including the Castle PHP SDK:
 composer install
 ```
 
-> **Note:** `composer.json` requires the released `castle/castle-php` `^4.0`,
-> resolved from the SDK's Git tags via a VCS repository. Once `4.0` is available
+> **Note:** `composer.json` requires the released `castle/castle-php` `^4.1.1`,
+> resolved from the SDK's Git tags via a VCS repository. Once `4.1` is available
 > on Packagist, the `repositories` block can be dropped. If Composer hits a
 > GitHub API rate limit while resolving it, add a token:
 > `composer config --global github-oauth.github.com <token>` (or `gh auth token`).
diff --git a/composer.json b/composer.json
index fa08ff7..f71fce2 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
     "php": ">=7.4",
     "ext-curl": "*",
     "ext-json": "*",
-    "castle/castle-php": "^4.0"
+    "castle/castle-php": "^4.1.1"
   },
   "require-dev": {
     "phpunit/phpunit": "^9.6"