Sounds like you're sort of in my boat. I have four kittens too, but with two big juicy ones, one medium one, and one true runt with exceptional problems getting on a nipple.
Now, a few things could be happening, and maybe you can give more details to narrow it down:
1) the runt is not adept at finding or staying on a nipple - you might want to see my problem as described over here
, as I've gotten excellent advice and clues.
2) the other kitties are slapping away the runt in their effort to get better traction on the nipples they have, or are actively taking the weaker runt's nipple away from it and pushing it out.
I've seen my two juicy ones do this to the smaller but not runt one. I adjust the juicy ones so they don't take up as much area around the remaining nipples, and guard the space around the remaining ones so that the others can mosey up and latch on as well.
3) Maybe the runt is trying to get to the treasured "rear nipples," which I've been informed have the best and most bountiful milk. If it's picky that way, maybe some time puttingit intentionally on one of those nipples and relocating one of the juicy ones onto a less favored nipple. The runt needs the extra boost.
4) the runt simply can't find the nipples easily (but is not experiencing the problems I'm having with my runt)
You can clear away the hair that covers up a nipple and hold the runt up to it so that it's easier for it to find the nipple.
If it still seems to be lagging and either not gaining weight or is actually losing weight, start supplementing with or completely bottle feeding it with KMR. Hopefully, when it is supplemented, it will gain the strength to hold its place amongst its siblings.
Another thing I've started doing is purposely putting my two shy/smaller guys on the mother and keeping the juicy ones away for a period. Since I know for sure that the juicy ones have ZERO problem finding and suckling any nipple within a ten mile radius, I have no worries that they'll get their feedings later.
But I do protect and watch over the period during which the two smaller ones nurse, making sure that they feed completely, and fall off the nipple voluntarily. Sometimes this requires that I sort of hold the mother in place. My momcat is prone to just jumping away when she's tired of nursing, but before the kittens are done. Also, I make sure that the kittens are TRULY ready to stop, and not just fallen asleep at the nipple. If they doze while nursing, I nudge them or stroke their back lightly and usually, they rouse up and start suckling madly again. :-) It's when they no longer do that that I know they are fully sated.
At that point, I allow the big juicy busters near the nipples to get their meal.
Hope that helps you a bit. I know it's worrisome to see one baby left out or lagging behind. It's important that they get enough food during this most crucial period, and that they gain weight on a normal schedule.