Because it still bothers him, and that's what worries you.
Of course he's going to talk to you about her; you're his girlfriend. Four years is a long time, especially when you're young. She was a big part of his life, and, in one sense, always will be. People always talk about their old flames to new flames whether we really care about that boring-ass bullshit or not. It's one of those necessary evils of a relationship, like cleaning up your girlfriend's vomit when she drinks too much and yaks in your car.
There's a limit, though. You don't say how long ago their relationship ended, but timing is everything. If it just happened within the last six months or so, the wounds are still fresh and it's perfectly normal for him to lick them. If they broke up three years ago and he's still bitching about her, then yeah, not good.
Oversharing is another problem. Most people don't mind discussing exes, but few care to hear every sordid detail of your relationship or personal, private information about your ex that you shouldn't be sharing. Also, no one wants to hear you go on and on about your ex being a crazy bitch or whore or asshole or liar or whatever else you like to call them. Not only is that tacky and inappropriate, but it also makes us wonder what you'll be saying about us after our relationship ends.
I don't know if your guy is fixated on his ex or not, because I don't know the timetable. People have to vent for a while before they move on. Regardless, if what he is saying about her or their relationship makes you anxious and uncomfortable, you have every right to say so and ask him to stop. It's a reasonable request, and a common one. You've been patient about listening so far, and now you would rather he not talk about her so much or overshare, because it makes you worry that he is still hung up on her. Their relationship is the past, and you would prefer to focus on the present and future of your relationship with him.