I am pregnant. My first day of my lmp was the 4th of January. I had sex with my boyfriend on the 16th of jan. and I had with someone else on the 25th of jan. I'm trying to get an idea of which one I am pregnant for. Just a guess, my boyfriend

That IS a dilemma. The best course of action is a blood test when the baby is born.

If you ovulate within the norm, I'd say your boyfriend... if you happened to ovulate late in your cycle, the other guy. Odds are it is your boyfriend's but, as ChihuahuaMomma said, a DNA test afterwards would let you know for sure.
They will likely do a scan for dating purposes and that will give you a better idea as well... although it is not exact, but it might ease your mind a little.