The joy of babies! Once you think you have their schedules figured out they change them on you! For years we have been hearing that adding cereal to our babies bottles will help them sleep but recent studies have shown that this simply isn't true and it can actually lead to overeating and obesity. Here is a great article from Dr. Greene who is a noted expert:
Your baby is probably just going through a phase or growth spurt and may need a few more middle of the night feedings until he passes through this one. When my daughter was two she was still getting up in the middle of the night and wanting a sippy cup of milk and she wanted it warmed up...ugh...I thought that stage would never pass but it did!
Here are a few great articles I found on getting your baby to sleep through the night and maybe you will find something in here that will help.
The main suggestion I have for you now is focused on getting you the sleep that you need! Is it possible for you and your fiance to take turns getting up to feed the baby? I know a lot of new moms are hesitant to ask their spouse to help out when you are staying at home caring for the baby (I am assuming, I apologize if this isn't the case and you are actully working too) and he has to get up and work but if you only had to get up every other night it would make a world of difference to you. Before I got pregnant my husband and I discussed how we were going to handle this situation and he agreed to take every other feeding from the time we brought our babies home from the hospital until they quit waking up in the middle of the night. They are afterall, his children too and it really helped him bond with the kids and to appreciate what I was going through. If you are working as well then I really believe he needs to share in this duty until this phase passes....which it will even though it doesn't feel like it right now!
Another thing that helped us was creating a plan. We put the bottle in to warm and while waiting on it we changed the diaper so we didn't have to do it after they fell back asleep and risk waking them up again. Also, keep the lights low and resist the urge to talk to your baby. Too much stimulation will wake him up further and possibly keep him awake and then you are stuck up with him! Pacifiers were also life savers for us and at 4 months old it may be worth a try even if he hasn't taken to one before now.
I hope one of these articles has something in it that will help out. Just keep telling yourself that this too shall pass :-) Good luck and enjoy your baby!