Hi Toni,
I'm sure someone who has all 4, can give you a better answer. I keep hearing 2 is the best, which I don't have. I know a lot of people have said 3 wasn't as good, not as intense as the prior 2, but honestly, I really like both 3 and 4. If you are looking for something that goes at a good enough pace where you can get some cardio work done, while at the same time slow enough where you can really focus on the muscles you are using, and improve your kickboxing techniques, then I don't think you can go wrong with either 3 or 4. I don't have a problem working up a good sweat with any of them, and actually like the slower pace now and then to really work my muscles and work on technique. I also love the way they both work my abs.
Iris