Funny you mention the GP 4000 because I got a flat this morning in one. They are nice tires but I need me some skins. I ordered some ultras.
On a similar note and because I am jinxed I checked the mtb today and the rear tire was flat. I rode it on my block last night for 5 minutes for gods sake. I took off the tire, which was quite a feat. I found a small hole, patched it. Now I can't get the tire back on. Any recommendations would be welcome.