If it is a digital machine you can use virtual tracks which live "under" each track. These comes as a standard feature on most machines. You can then take your mix of the 4 tracks and bounce then to a stereo or mono track (this means to play all the tracks together and record them to a stereo or mono mix). This then frees up more tracks and allows you to play back the mix you have just bounced and record another 2 or 3 tracks along side.
This is a non destructive feature meaning the original tracks still exist in their original format and can be remixed at a later date after an initial bounce.

If it is an analogue machine you need to bounce your 4 tracks to a mono or stereo mix and then record over previous tracks. This is destructive and means you are stuck with the mix you have generated