I have owned 2 RZR's, one standard and one S model...both were a blast, but for the dunes...you can't beat the RZR-S.
Do you want a 4 seater for 4 adults or 2 adults + 2 kids? If it is for kids, Vent racing makes a cage extension for the RZR that allows you to put a bench seat in the back, but the leg room is a little limited...good for a kid or two, but not much for adults back there.
If you want it for 4 adults, the RZR-4 is the best option. It will ride 10x better than a Rhino or Teryx on the sand (and will outrun all of them too), and will handle MUCH better than the 2 seaters with the 4 seat conversions.
But Polaris is supposed to announce their 2011 model lineup sometime next month...rumor has it that there are 4 new/redesigned models in the RZR lineup, and possibly an 1100cc motor to compete with the new Can Am Commander 1000....so you may want to wait until their press release.
If you aren't set on getting a new one, check out some of the UTV forums...you can snag up some well built used machines for some big savings...take a look in the classifieds at: