If he really does treat you the way you stated he does, and that your sex life is great, I wouldn't worry too much about it if he is looking at random images/clips of girls he will never meet/know, guys are just guys and somehow separates sexual desires from emotional attachment. He just needs to get off, something monkeys and gorillas do as well. It's when he has the emotional attachment to some other girl, that should concern you.
But if he really is spending 4 hours watching porn at a time, I suggest you look into it or talk to him about it, I don't think a guy can masturbate for 4 hours unless he has some insensitivity issues =/. Not to cause an alarm, but you would have to consider the possibility that he is doing simply more then masturbating/watching, if he spends that long.
As the above poster, there is no big deal if a guy watches porn of images with girls he won't remember seeing, but if he is interacting with webcam girls on a constant basis, I too would agree that borderlines cheating.