I stayed in the 4*. It is basically one large hotel but the 4* and 5* are on different sides. You all use the same facilities and restaurants though, depending on what package you go for. Eg, I was 4* but could eat for free in 5 of the 7 resaurants, I had to pay a supplement to eat in the other 2, 5* customers can eat n any of the 7 for free (I think).
Also the 5* have superior rooms with spa baths and some with roof top private pools.
Staying 5* gives you some free access to the Spa, you can still use it if you are a 4* customer but you pay extra for some things.
There weren't really any exercise classes to speak of, it was mainly spa treatment, massages, sauna's, steam rooms, beauty treatments etc.
For the amount of time I actually spent in the room and the fact that I didn;'t really fancy the other two restaurants anyway (one was Vietnamese), I was more than happy with the 4*.