Sounds like a pet, so this forum is a good one. There are others but...it is up to you.
Some things kind of depend on the breed, especially what size the breed is. Smaller rabbits seem to mature a bit faster, I think.
I know that many meat rabbit breeders wean at 4 weeks. I usually wean at 5 to 8 depending. Rabbits in the wild usually wean at around 4 weeks because the doe is usually having another litter at that time.
I personally would not supplement with a milk replacement at this point, as long as the bunny is thriving. You need to just have quality pellets and hay. They need chew ro wear down their teeth and to have a lot of fiber that hay gives them, but too much alfalfa can make them fat.
Rabbits have two different kinds of poop. One is round and dry. The other is softener, moist, and will stick together or on the fur...the rabbit eats this and should to stay healthy. As long as it has both types, it is normal. (Diarrhea is not good.)