If the ability to add counters resolves when there are already the maximum number of counters on it, any counters over the maximum are simply not added.
This creature enters with four +1/+0 counters on it.
At end of combat, if this creature attacked or blocked this combat, remove a +1/+0 counter from it.
, : 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.
Loses a counter even if it is affected by a Fog-like effect which prevents it from dealing damage.
, 
: 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.