Would 6 months be enough by itself? No, of course not. But the thing is, it's not really all by itself.
Unlike 2>3 which had several major changes to the overall mechanics of the game, 4.0 seems to be more in the nature of a lot of tweaking to the rules, more so then 3.5, but enough to still be recognisable. Add in the fact that a lot of the new changes have already previewed earlier in other products, and I'm sure they already have reams of data on how the mechanics worked and incorporated them correctly. (Fingers crossed)
I believe they have exactly the product they want, the external playtesting is there to make sure (As someone mentioned previously) the rules are clear enough, and to help spot any glaringly obvious problems that internal playtesting has missed.
That said, I'm sure there will be SOME sort of problem. No matter how much playtesting anything gets, it's never enough to get rid of every single problem out there.