Go to steampowered.com and log in. Click the drop-down (Usernames' account) in the upper right corner and then Account details. The last four digits of your CC should be listed under "Your Steam Account" on the right.
Go to steampowered.com and log in. Click the drop-down (Usernames' account) in the upper right corner and then Account details. The last four digits of your CC should be listed under "Your Steam Account" on the right.