O owned a 4th gen and have just ordered an Ultimate, so I cant give a great judgement until it arrives, but on the aesthetics....
In general the pearl on the Ultimate looks much better. It doesnt stick out and over the top of the silver ring like on the fourth gen. The minute hand was ever so slightly too short on the 4th gen, hardly noticeable. The clasp on my old 4th gen seemed a little flimsy, but from what I've heard the UPO has a very good level of fit and finish
However,
The UPO has the wrong Omega symbol (it has little feet that kick up, on the 45mm they shouldn't have the upturns) The UPO was having reports of the crown stem threading. Now this hasn't been reported for a while so hopefully they have fixed this problem, but it's something to be aware of.
So neither are perfect, so it all depends on what kind of flaws you can live with, and what size you want. I loved the 4th gen, only reason I sold it was because a friend wanted it so much. So I am replacing it with an UPO to get the next size up, and in my opinion it is the better of the two watches