Rabbits have two kinds of poo. Fecal matter is the little round pellets that I'm getting waaaaay too used to accidentally stepping on.
Luckily it's when they're dry and I can vacuum those ones up really easily. Cecotropes are the squishier wetter ones, and those ones are full of nutrients. As gross as it sounds, the rabbit should be eating them to get the nutrients it missed the first time round. They should be more like a bunch of grapes stuck together though (except obviously smaller), rather than very wet. If he's only four weeks old and you're worried about runny poo, I would take him to the vet and see if you can get something to help him - the paragraph I found about this says:
"At the first sign of runny stool in a baby rabbit, off to the vet! Treatment for diarrhea in baby rabbits will probably include subcutaneous fluid thereapy, and administration of oral probiotics. Lactobacillus acidophilus
powder (NOT yogurt, which can make the problem worse) suspended in clean drinking water and carefully administered via syringe seems to help foster a healthy intestinal environment and may even soothe intestinal inflammation. A very small amount of a clay-based product such as Kaopectate can help solidify the stool and stop the cycle."
Unless anyone knows better? But I know I was going straight to google to get exact specifications of what bunny poo should look like yesterday when Holly did some wet ones outside his cage. XD Luckily it was just cecotropes, he doesn't usually come out of his cage at that time and I hadn't seen them very closely until then.