Do you breastfeed or bottle feed? I think with breastfed babies this is pretty normal. Both my kids were sleeping through the night before or at 4 months, but I also knew how much they were eating, etc. I think the feeding technique makes a difference, but I'm not sure, I didn't breastfeed so I may be wrong.
Also do you have her crib mattress tilted? That will help with the reflux. You can either hook one end higher than the other or just shove a towel or blanket under one end of the mattress so she's at an angle.
If you want the advice I got from my pediatrician to make it happen let me know! It might work both ways, but I know breastmilk gets digested quicker and they might wake up earlier b/c of it! Hang in there! The first 6 months are the hardest!