There are two options, actually:
1) Connect the audio outputs of the 4k to the audio inputs on your soundcard using standard audio cables (depends on what connectors your soundcard uses), check levels and hit record. Easy, but the quality of the recording will depend largely on the quality of your soundcard... Recommended for long beats and complete songs.
2) Install Ak.Sys on your PC. Set the 4k sampling source to "main outs" (under the "Record"-page), sample your beat, then simply drag the new sample from the sample listing window in Ak.Sys to the desired location of your hard drive. Remember, if recording in 24/96 you will have to do some conversion before burning an audio CD (if that's the plan). Recommended for shorter beats/songs.
You'll find the latest version of Ak.Sys and USB-drivers for your 4k here:
http://www.akaipro.com/global/download/aksdload.html
Good luck!
Arne S.