I'm new to kittens too, and from what I've read, when a kitten is full, they look like they've got a golf ball in their belly. Now, I have four kittens, and this varies among them. Two of them, after feeding, do look like they've got golf balls in their bellies, and they are taut and hard to the touch.
The other two never seem to get taut and hard; they get fuller, but never what seems completely full to me. One of them has a problem finding and keeping on a nipple, so what I do is take the two that never seem to have enough and create "dedicated feeding times" to them, after the two hogs are finished. :-) I put them on the mom cat and lead them to the nipples and keep the other two away. This results in them getting a bit rounder in the belly, and I think they appreciate the time to eat with no competition.
So, maybe that can work for you, and the kittens will have more food to fill their bellies. I guess you could break it up into two kittens at a time. Also, you could try your hand at supplemental feeding with KMR (I have not yet succeeded in this myself, it's hard, and they prefer the mom's nipples).
I think the time to really worry is if they are wrinkled in the belly...but it can't hurt to make sure they are fed until they fall off the nipple of their own accord.