I would say try 3.5oz and see how he does.. I honestly can't remember how much DS was drkinking, but I know it was way more than what other people's babies on the bump were drinking and I was panicking that I was over feeding him.. when I asked the Ped about it, she laughed at me and said FEED him if he's still hungry.. that I didn't have to worry about overfeeding, he'd spit it up if it was too much..
Also, keep in mind that around 1 month there's a growth spurt, so it could be that he'll just eat more for a few days, and then you'll find maybe he'll just go back to the 3oz..
Basically, from my experience.. I followed his cues.. if he wanted more, I fed him.. if he didn't take it, he didn't take it..