Redirecting URLs using RewriteRule RewriteEngine in Apache .htaccess

Using RewriteEngine's RewriteRule within Apache's .htaccess, I want to redirect urls containing this:
user_screenshots/4

To this:
user_screenshots/saves4/4

So for example, this url:
http://www.vizzed.com/vizzedboard/retro/user_screenshots/47040/Pokemon%20Yellow_Dec29%2021_37_35.png

Would get redirected to this:
http://www.vizzed.com/vizzedboard/retro/user_screenshots/saves4/47040/Pokemon%20Yellow_Dec29%2021_37_35.png