You mean an ultrasound? No that is medical grade equipment - there's no way you can do that with a phone.

I'm not sure if there's an app for that or not, but I know if you're only 4-6 weeks you wouldn't really see much.. Maybe a little blob. It might do more harm than good, if you don't see anything (which would probably wouldn't this far along) it might scare you. I had my first ultrasound at the dr office around 9w and even then it was just a little bean looking thing on the screen and I'm sure that is much better technology than any app you'd find.

nurture, has good articles too