Congratulations to your new bunny! 4 weeks is very, very young though, bunnies shouldn’t be taken from their mums until they’re 8 weeks old. It’s really sad that people will sell a bunny so young.
For litter training you put some poop and some paper with her pee on it in the litterbox. It’s best if you can place it where she/he has peed once, because they do tend to like to choose the spot themselves. If the litter box is in the wrong place, the bunny won’t acknowledge it. What’s a right and a wrong place, only a bunny can tell.
As for bonding, it’s great that she/he comes up to you, but it’s best if you don’t stretch your hand out towards her/him. Let him/her explore you without you trying to touch him/her. For starters you could even try not to look directly at him/her (very hard, bunnies are so cute. But look directly at is what a predator does before it starts chasing the bun to have him/her for dinner, so bunnies tend to be wary of being stared at). You can even lie on the floor and let him/her use you as a jungle-gym!
Food wise you should offer ad lib (=as desired) junior rabbit pellets and hay, preferably both alfalfa and normal grass hay. Water of course, but no vegs and no treats until he/she’s older (12 weeks). You can hand-feed some of the pellets though, so she/he learns to associate your hands with nice stuff.
We’ll be looking forward to hearing a lot more about your bunny!