Four children are playing on a hill of sand.
The children are at the beach.
Children can be more or less than four.  Just because children are playing on a hill of sand it does not mean they are at the beach.
exists e1 x1 x2 x3.(on(e1,x2) & of(x2,x3) & n_sand(x3) & n_hill(x2) & Actor(e1,x1) & v_play(e1) & n_child(x1) & card_eq_4(x1))
exists p1 x1 x2.(n_beach(x2) & n_child(x1) & at(x1,x2))