Clockwork Steed

This creature enters with four +1/+0 counters on it. This creature can't be blocked by artifact creatures. At end of combat, if this creature attacked or blocked this combat, remove a +1/+0 counter from it. {X}, {T}: Put up to X +1/+0 counters on this creature. This ability can't cause the total number of +1/+0 counters on this creature to be greater than four. Activate only during your upkeep.