Although there are some old thread on this, there does not seem to be anything recent.
V3 is still my favourite even after extensive testing of 4 and 5. I thought about 4 but afaik its not receiving any new features and I see far too many odd cosmetic concerns even now
3 just seems perfect as all that's required is a forum nothing more. I've fixed my localhost copy so its working OK with php5.5. and mysql 5.6 the latest one seemed OK apart from a couple of minor issues. Impex I had to spend a bit of time 'fixing' unfortunately.
FWIW I've had the licence for a long long long time but never really used it, but the time is approaching where I will be.
Any thoughts please ?

I guess its just security fixes I'm concerned about really, although having said that while there are so many IB sites on 3.x afaik I guess we may receive said updates ?