Its highly unlikely that an ultrasound that early is off by 4 weeks. i'd confidently say its your boyfriends baby.
The ultrasound is more dependable I believe your bf is the daddy and congrats.
Plan B doesn't stop you from ovulating, and you're very lucky that it didn't kill your baby. Having sex AFTER taking the plan B pretty much defeats the purpose - as it's meant as back up birth control within 48 hours of having unprotected sex. Also, PPs are right, the ultrasound can't be that far off and your boyfriend is the father.