Many (most?) won't or can't (depending on your circumstances) make the break completely away from 3.x. There are enough features still in v. 3 NOT in v. 4... OR you simply more efficiently done - so far as you currently work - in v. 3.
That said, I'm making the break and actually enjoying it.
To add to my joy...I gave up my old faithful PC and made a new PC purchase (Quad-core, 8 Gig RAM) for 60% the cost of my previous PC. Whoa Nellie!!!!! Thanks to all who helped me focus on certain considerations and specs (especially Jim Dean). I didn't do the RAID setup - I'm sure that would help even further...but I am soooo pleased with the way Logos works on a new machine.