Like the others here I can compare GP4000S to the Pro3. Here are the pro's and con's as I see them -
GP4000S - Great grip in the wet, reasonable puncture resistance. I puncture about once every 2,000 miles riding South of London (lots of flint washed into the road when it rains). The drawbacks are that they feel rough and unforgiving and that the sidewalls aren't very durable.
Pro3 - Great grip in the wet (probably better than the Conti's) and mediocre puncture resistance but they feel much better than the Conti's. The problem is that using them here in winter destroys them quickly (very soft rubber compound), they're fine in the dry though. I have some Pro4's on the way which I plan on racing our winter crits on so will report back if this thread is still visable by the weekend.
I train on GP4000S and race on Pro3's (or in very good weather Vittoria's which have awful puncture resistance and equally awful wet weather grip but ride beautifully in good conditions). I'm perfectly happy racing Pro3's through the winter crit series so while they're a little delicate they're not that bad (it's Kent/Surrey country lanes that destroy them).
I second all of this entirely. (And I'm also London-based, where southern excursions also result in my Pro3's getting punctured/eaten!). I race Vittorias in perfect weather, Pro3s in anything else, currently have 25mm GP4000s on for winter training (which have almost all of the grip of the Pro3's but much better durability, even more so on both counts in 25mm guise). My GP4000s are near the end of their life and I've just purchased Ultremo DD's in 25mm to try something new. Been on one ride on them and impression is favourable: the sidewalls are noticably more substantial than any of the others, haven't tested the grip but I expect it to be ball-park there. The only thing.... the 25mm DD's barely measure 23mm mounted! They're seriously thin, thinner than e.g. 23mm Open Corsas which had just come off the rim. Not cool, as half of the reason to run 25's is to be able to drop pressures without risking pinch flats, which I'm now reluctant to do - shame, as these are almost the perfect winter tyre. In contrast 25mm GP4000s do actually measure 25mm, and 25mm PR3s run super fat (great tyres, fast comfy and grippy, but I don't have clearance on one of my frames).
Would be very interested to hear how you go with the Pro4's.