I have been going out with my girlfriend for 4 months and she has told me she likes sumone else but always says she will be mine forever.. yet I just noticed on her instagram it has something related to her in a comic about her "loving" someone else and of course er comment says true story... what do I do if she likes sumone else

Very simple' if you love her too. Then, its about time to let her free.

4 months is a very short time. Let her go and if she really cares for your relationship, she will fight for it. Let her decide what she wants. If she chooses the other person, then you know where you stand.