I have recently been messing around with my boyfriend and there is a possibility his cum got on my vagina my period is 4 days late could i be pregnant?

Yes but only if . . : If you engage in sexual activity w/o protection, then yes, you can potentially get pregnant. So if you don't want to get pregnant, use some form of contraception, typically hormones, eg pills, patches, shots or implants. But these don't prevent sexually transmitted diseases (infections) so make sure he wears a condom each & every time, no matter what he promises or says about trust, love, etc.

Yes: There are many things that can make a period late, but pregnancy is probably the most common. Check a pregnancy test and you will know. It will already be positive if you are pregnant.