4 inches is smaller than average, but it's hardly "tiny". 5.5 inches is average, so you're only 1.5 inches below 'normal'. There is no reason why, if you know how to use it, you cannot give your girlfriend an orgasm during intercourse. Spare a thought for guys with 'micropenises' that are under 2 inches when erect! The kind of girl who would laugh at you for this are either immature, and would probably giggle at a penis of any size they saw online, or they are spiteful sluts who most probably have a vagina like a wind sock.
My advice, wait until you're sure that your girlfriend genuinely cares about you, then when you feel like you're ready perhaps discuss your anxiety with her. I'm sure if she's at all reasonable she will be understanding and sympathetic about your self-consciousness. If she isn't I'd personally question with she's the kind of person I want to be in a relationship with.