Waiting is the wise thing to do right now. There are too many different 4.0 hack methods / guides being posted right now, and none have seen enough testing to convince me they are safe or even do what they claim to do (trucha and etc.)
That said, the new features of 4.0 are not a compelling reason to upgrade. The only real reason is Shop Channel and WiiSpeak compatability, but I personally don't care about that.
I'm on 3.3v1 and have no problems doing anything I please with my Wii, and I like it that way.