How ddj-rr is a 4 channel mixer ?

These switches here:

When you switch over, the controls all work on that other deck - you simply move the knob or fader to "pick up" the software position of that control.

But can you play 3 or 4 tunes together ? at the same time ? or do you have to switch between 1/3 and 2/4 ?

Yes - they play simultaneously. Because the DDJ-RR is acting as a hardware controller for the software mixer within rekordbox, you are able to use two sets of channel controls for four channels. The audio is mixed in software and output to the "master" directly.