Also have put small amount of cereal in bottle with formula for 7pm feeding and he drinks it all...
I really advise against doing this. Cereal in bottles can be a real choking risk. And actually there is no evidence that that will even help a baby sleep better but evidence that it can disrupt sleep more. He really is too young for cereal and it could be causing discomfort and exacerbating NWs.
Looking at your routine and posts I think the NWs are a combination of hunger/snacking and OT from the short naps.
This is a time when a lot of mums start to doubt their supply as this is when the breasts stop storing milk and you produce on demand. Like you a lot of mums start to top up with formula but every oz of formula you give is an oz your breasts don't knwo to produce. there is a HUGE growth spurt at 16 weeks so it is important to feed, feed, feed when it hits and not supplement very much at all or it can really affect BFing going on.
Those amount look totally fine to me for a BF baby. And it is not unusual at 15 weeks to still need 2 NFs. Pumping only tell you how much you can pump not how much the baby is getting. babies are much more efficient at getting milk from a breast than a pump so its likely your baby is getting more than you are pumping.
Did the NWs start when you introduced the formula?
If he comes of the breast happy and satisfied then you can't say that you have low supply and should stop the formula.
I agree with this. I would stick to 3.5 EAS and not 4 at this age.
To work on the NWs I would pick one to work on. So maybe the 3am one? So feed at 1am and make sure he take a good feed if you can and then resettle the 3am wake up however you can. Have you tried wake to sleep? If he feeds at 1am he should be able to last for 3-4 hours without a feed. And it may help shift the hunger to the day time.
But I think you arae doing great. I think adjusting your expectations a bit may help. 2 NFs is ok for this age. so 1am and 5am is fine s work at eliminating the 3am feed. and then once you get through the GS work on the 1am feed and at least then you'll get a stretch from 10-5 which is normal for 4 months.
HTH