Yes, only zero to four. The numeric system we humans use has a base count of zero to ten, this is because we have ten fingers. Ponies have four hooves, so logically their numeric system would have a base of five.
0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. would be 0, 1, 2, 3, 4, 10, 11, 12, 13, 14, 20.