Update documentation version link in readme

Author: Slund

README.md

@@ -60,7 +60,7 @@ We highly appreciate you sending us a postcard from your hometown, mentioning wh
 
 ## Documentation
 
-You will find full documentation on the dedicated [documentation](https://spatie.be/docs/laravel-data/v3/introduction) site.
+You will find full documentation on the dedicated [documentation](https://spatie.be/docs/laravel-data/v4/introduction) site.
 
 ## Testing