Sleeping through the night is a developmental thing, it has nothing to do with how much he eats and when, and when he goes to bed and/or how much he sleeps during the day. And he's breastfed. that doesn't mean he isn't getting plenty full, it means breastmilk is digested much faster (which is a good thing), so the baby may want to eat more often. It's also a matter of comfort. Some babies wake up to nurse not because they are hungry, but because they enjoy breastfeeding.
I don't know if he is breastfed exclusively or not, but if he is, wait to start solids. You'll know when he's ready, and it won't have anything to do with his sleeping patterns. Also, stop feeding him cereal in the bottle. That is the absolute worst thing you can do. I work at the health department, and I see so many moms doing this. This can lead to obesity, as they are just getting a ton of calories at one time, just pouring in their little mouths, also it is a huge choking hazard. What comes out of that bottle is much thicker than what the baby is used to. This may seem to help the baby sleep through the night, when all it is really doing is giving your baby a false sense of being full, because it takes so long for him to digest. It just sits on his stomach.
He's 4 months old, don't fret over him waking up a couple times during the night...my son didn't sleep through the night for well over a year! Each baby is different. Enjoy your alone time with him at night, watching him fall back asleep. It's precious and doesn't last forever, though you will notice, you will always have nights where your child will wake up in the middle of the night, no matter how old! Cuddle with him when he wakes up, and nurse him back to sleep. If he's sleeping with you, you won't really lose any sleep, as all you have to do is roll over and nurse and you will both fall asleep within a minute or two. Don't worry about your milk not being enough. It's more than enough. Babies all over the world live off of breastmilk ALONE for the first year of their lives.