To drop a caller you hit the conference button and hit the red phone icon next to the person you want to drop then you can add another caller. You are limited to a 3-way conference call using the native phone app, HOWEVER, if you have the 4th person call you instead of calling them and you hit "hold+answer" you MIGHT be able to merge the calls and have a 4-way conference, but I have never tried it before it's just a guess.