I've written elsewhere here that bang for buck-style Motion 5 is the best bargain out there for motion graphics. However, you already have Motion 4 so the question really becomes, are you using it? I skipped Motion 4 and off the top of my head (just getting up in the AM too) I'd say you are not getting a lot by moving from 4 to 5 except some additional content and 64-bit. Now, with all my praises for Motion 5, I still think the downside is... it's slow.
If Compressor 3.5 is working for you I don't see any real reason to go to 4.