4-5 oz per feeding sounds about right :) You also have to remember that just because they're babies doesn't mean their feeding habits are any different. Think about your own habits during the day. You may have a huge breakfast, then a small lunch, several small snacks during the day, etc.Your baby eats the same way, and if he's breastfeeding in addition to bottles, you don't know how many oz he's getting at the breast. To be honest, I think we are too consumed with how much our kids eat instead of taking the more natural approach of on demand. We eat when we're hungry, right? We're not stuck on a rigid schedule and forced to eat when we're not hungry. Yes, there are certain situations where a baby should be woken up to eat, but at 4 months, you're safe to let him go if he's not hungry.
As far as formula is concerned, you have to weigh the pros and cons. Formula is NOT the best food for your baby; your milk is, as you already know. And have you ever tasted formula vs breastmilk???? It's no wonder babies don't want it!! He's too young for rice cereal, as his digestive system isn't mature enough yet. Research has also proven that cereal does NOT help babies sleep through the night. Sleeping through the night for a 4mo old is about 6 hours, so take a look at his sleep habits as well.
Just a few cents from me.... Enjoy your son :)