if they stayed with mom, who was litter box trained, until they were at least 8 weeks old, they are already trained. just put them in it so they know where it is.

if they have never seen a litter box before, just put them in the box , a lot. after they eat, whenever you see one of the other kittens using it, when they have an accident.
also, after an accident clean it up and put it in the box. they go where they smell them selfs.

and, 4 cats should have 4 litter boxes, in separate rooms, far away from each other.
they will use one box for now, but by 6 months old, you should have all 4 boxes set up.