My cat can't feed her kittens enough. How much should I supplement?

I have a fairly young immature cat with four kittens and she doesn't quite have the hang of raising them. She can only seem to nurse two of them at a time because she won't stretch out long enough for all of them to reach` a nipple. One of them is very small and weak and has significant trouble with nursing and another just isn't very smart. She will nurse two of them for a couple of hours and then leave for a couple of hours. Generally, it is the same two that get nursed each time, and the other two go without. Consequently, I have been supplementing with formula, however; I am concerned that too much supplementing will stunt the rooting reflex. I am currently supplementing the two weaker ones every two to three hours. Is this too often? What do I do if the smallest one never nurses? Do I also need to stimulate the bowels or can I leave that part to mom?