I know 4.x is working better. I operate 9 forums and 5 of them are on 3.8 and 4 of them are on 4.1.12.
The forum with Classifieds is a very large forum that's been in operation for 14 years. The folks there are not at all happy with changes. 3.8.7 works just fine and we see no real benefit in upgrading to 4.2.
5.0 is another horrible mess, even worse than 4.0 was in the beginning.
I can see moving some of my forums to 4.2, but don't see the benefit at this point. I realize I can not stay with a EOL product for 3 more years. I just don't see any thing better "right now."