Nups, he's definately gay now. Gay porn is particularly nasty because it launches gay particles straight into the brain through the eyes, anyone caught watching them will turn gay eventually. It might not happen immediately, but it will eventually happen. There's no hope for him now. The only thing left is to leave him to his new gayness and find someone straight who isn't infected.
Okay, sorry about all that, I'll be serious now.
Sexuality has to be one of the most confusing things for people on the planet, it's so varied and different from person to person that the only way most people can handle it is by labelling it - Gay/Straight/Bi/Whatever. I don't think this is an adequate way to measure it, and it will just lead to confusion and problems like what you are having.
Every person is unique in themselves and this includes their sexuality. Most people are hetrosexual, because this is the human's way to procreate and continue to exist, but lots of people are not. I've met people who are straight, gay and bi. I've met women who like looking at other women and their bodies, but won't take it any furthur. I know a few men that are bisexual for the same reason too, but they don't find man on man intimacy enjoyable. Even those that are hetrosexual still have various levels of enjoying or not enjoying the look of their own gender. I check out other guys occasionally, but in a non sexual manner. Even the ones that call themselves gay have various levels of affection for those of the opposite gender. Some like them, some don't. It's all very subjective and unique.
In your situation though, the best thing to do would be to let your boyfriend open up by creating a safe space for him where you won't judge or evaluate him and his views. He's probably uncomfortable about himself, but also curious at the same time, which will make it even harder for him. He's probably straight, especially if you have been going out for almost four years, but that doesn't mean he doesn't find homosexual intimacy interesting. Instead of asking him about it directly, which will just cause him to retreat, get to know him better without direct questions, share more intimate moments without an agenda, and just grow together as a couple. Before guys can share what's really important to them there needs to be a very high level of trust and emotional safety. If you really want to find out what's up with the gay stuff, you'll have to work on the overall relationship.
So after all that I would say: Yes, a guy can be completely straight and still watch gay porn.