@Role up,
Sex is very important for most of us, and in most relationships couples have sex at least one or two times a week. I think it is completely reasonable for you to be frustrated. I would not stay in a relationship that did not include regular good sex... what's the point. If you aren't having sex regularly, why not just be friends?
There are asexual people who don't like sex. And I understand that they can form meaningful intimate bonds that don't involve sex. There is not problem with that... as long as both people are ok with it. But that isn't for most people. If you have a normal sex drive, being in a asexual relationship would be incredibly frustrating.
If this were me, I would let my girlfriend know very clearly that regular sex is very important to me. I would talk to her about whether things can change (and change quickly).
If the answer is no. I would end the relationship and find a partner who enjoys and want sex as much as I do. Having to beg for sex is a very sad place to be.
Yes, sex isn't everything. An intimate relationship should absolutely be about caring, and emotional support and friendship and all the other things that make human relationships.
But sex is important. If you are not having your sexual needs met in a four year relationship, it is time to move on and find someone who will make you happy.