The warranty aside, the Panasonic is a better image than the Epson. For the money, the price is good for the Panasonic quality. But, the quality difference between the 8500, 4000, and 8100 is not night and day - it is subtle. You WILL be happy with any of the projectors, and if you aren't happy with the projector, then you likely won't be happy with any of them, because they are all very close in quality. Noticably better, but not worlds better from one model to the next.
If your budget barely affords you a 8100, then get the 8100 - you won't be unhappy, but the 4000 is better, and if your budget affords the 4000, then get that instead. Ditto on the 8500UB.