Oathsworn Knight

Oathsworn Knight's power/toughness is zero.
Oathsworn Knight enters the battlefield with
four +1/+1 counters on it.
Oathsworn Knight attacks each combat if able.
If damage would be dealt to Oathsworn Knight
while it has a +1/+1 counter on it, prevent that
damage and remove a +1/+1 counter from it.