Hi

Sorry your post was missed. At 4 months, LOs typically are awake for 2hr and by 19 weeks, that's increased again to 2:15 or so. That's probably the reason for the short naps - undertired initially, then overtired from short naps. Its quite reasonable for a baby of this age to still be having 2 night feeds, so I wouldn't worry on that front, though you may find he sleeps better at night once your daytime routine is a little more appropriate for him.
I'd hazard a guess that he sleeps longer in that first nap when he's been up earlier in the morning? I think he's extending his night sleep there. That's why increasing that first A time is crucial. You could also consider waking him at the 2hr mark. Increase the first A time from where it is now, to encourage him to sleep later in the morning so you've not got a short night with the early waking.
Here is some info and a calculator to get an idea of how much milk there should be in a bottle of EBM:
http://kellymom.com/bf/pumpingmoms/pumping/milkcalc/I'd imagine his routine would look a little like this:
6:30 - wake, eat
8:45 - nap
10:45 - wake, eat
1 - nap
2:30 - wake, eat
4:45 - nap
5:30 - wake from catnap, eat
BT feed
7/7:30 - asleep in bed