I was on v3.6 for the longest time and then I got an EOS M and needed to upgrade if I wanted to edit the RAW files. My mate had been telling me about v4 even before that but I hadn't bothered upgrading as v3.6 worked just fine. I'm glad I moved up now. Processing time has decreased and it seems a lot easier to get what you want. For me the absence of the fill slider threw me a bit but once I got used to the shadow and highlight sliders I wonder how I lived without it! Fill was so crude - it would just make everything lighter and didn't actually fill those shadows like you hoped it would. And recovery wasn't all that great either. I barely have to touch the exposure slider now (remember the old trick of underexposing then adding fill?).
In short if you do even a moderate amount of post it will save you time and stress. Make the switch.