Well, I did it. Down loaded MC 4 with MC3 still installed on my computer. Smooth sailing. MC4 has a nicer display, in particular, you can view your project by each bar instead of the weird breakdown in MC3. There are a lot more synth options. I haven;t really used it much, just explored a bit. It will find and open MC3 projects as if they are MC4 projects. The only thing I had to do to make it work is switch to MME drivers, after that, piece of cake. It automatically uses the same settings as MC3 (it asks you if you want them).
Definitely worth the $35 in my view.