Boundary Lands Ranger

At the beginning of combat on your turn, if you control a creature with power 4 or greater, you may discard a card. If you do, draw a card.

Pioneer Masters

"You only find out who was hunting who when one of you is dead."

Wilds of Eldraine

At the beginning of combat on your turn, Boundary Lands Ranger's ability will check to see if you control a creature with power 4 or greater. If you don't, the ability won't trigger at all. If the ability does trigger, it will check again as it tries to resolve. If you no longer control a creature with power 4 or greater (usually because your applicable creature is no longer on the battlefield), the ability won't do anything.