Hi there
yes I would expect a 4 month old to be on 3hr E but all LOs are different, I suppose the thing is to work out if your LO does need these additional feeds or if there is a snacking habit.
Does your LO have any signs of reflux or silent reflux? Refluxers often feed more frequently because it helps with the discomfort of the acid. Have you thought about this at all?
Try not to compare with other babies, I know we can't always help it. My DS was formula fed and I don't think he ever once took 6oz in the day that you refer to, he could take that amount at BT and at the DF time (although he woke for it) and perhaps once in a blue moon during a growth spurt, and he was probably much older than your LO by the time he could take as much as 6oz at BT/DF, I can't claim to remember exactly what he took at BT when he was 4 months but I imagine more like 4oz. My DS is not "the norm" on amounts of milk but he gained weight and grew absolutely fine. Again I can't remember the exact age (possibly 4-6 months or older) but I do remember for a good time mine taking around 2oz or just over, not reaching 3oz for day feeds. He fed every 3hrs. Whilst I'm saying try not to compare I'm also showing you an example of my DS to compare to! Because not every baby takes what is written on the side of the formula pack.
If naps are short anyway I wouldn't really worry about him waking hungry, I'd probably go for the nap without the additional feed before hand to avoid/break the F2S prop and feed him when he wakes instead. I suppose there is a slight possibility that putting him down on a full tummy for naps could be causing some discomfort (reflux or wind, gas) which is bringing him awake at the sleep transition rather than staying asleep.
If your LO really does need to eat that frequently though you can work an EASY routine around it, we adapt routines here all the time for LOs needs.