How many custom mixes the PSM600 can handle? I did see in the product description where it broadcasts on two frequencies, which seems to me (pardon my ignorance) to allow only two mixes.
You could certainly use the PSM600. Essentially you will need a separate transmitter and separate receiver for each stereo mix that you want. So for you, that would mean 4 complete PSM systems, each on its own frequency.
The PSM600 system is available on 5 different pairs of frequencies (10 total frequencies). So, (1) your 4 systems could take up the first 2 pairs of frequencies, or (2) your 4 systems could each have a different frequency set. Either way, you would still be operating on 4 separate frequencies. Obviously, the second method is a little better, because you have the capability to switch frequencies on any system if there is interference.