Hi M.,
First, congrats on breastfeeding. I did with both of my boys and remember worrying about whether they were getting enough milk. It may be that your son is actually getting plenty of milk, but is crying for some other reason. 4 month olds don't really need that much food. If your son is gaining weight at a normal rate, he's fine.
Do you alternate the breast that you start with? I would put a safety pin onto my bra strap to help me remember which I had started with the previous time. Do you need to wear breast pads to prevent leaking? (As an aside, folded men's handkerchiefs work well as breast pads.) If you are, then you are probably producing enough milk.
I found the lactation specialists at my hospital were helpful. Also, you could try calling your pediatrician for suggestions or your ob/gyn. Finally, if you're not afraid of their sometimes very strong opinions, you could call La Leche League. I found a lot of good information and support, but was a bit uncomfortable about the intensity. (As another aside, I goodsearched "breastfeeding support" and found a bunch of websites. I couldn't think of La Leche League's name.)
The best way to increase your milk supply is to drink a LOT of water and to pump between nursings. You may be surprised at how much you are producing. If you introduce formula, you may decrease your supply because your son may nurse less, and nursing is what stimulates your supply. The problem is that the cheap pumps really don't work very well. I pumped very effectively, but only when I had one of the expensive pumps. I don't know where you live, but in Santa Monica, there is a store called, "The Pump Station," that might have some suggestions for you. Also, if you have a friend who is also breastfeeding and pumping, you could consider supplementing with your friend's breastmilk, rather than formula. That wouldn't increase your supply, but it could keep your son on breast milk longer. (I know that sounds kind of weird. I provided breast milk for a friend's adopted son who had been constipated.)
Finally, if it doesn't work out, you've already given your son four months of breast milk only and that's a great start. Formula isn't bad and try not to let others (or yourself) make your feel bad if you end up supplementing. Lots of very healthy babies have received formula.
So good luck! E.