Whether you control a creature with power 4 or greater is checked only after your opponent has sacrificed a creature. If a creature you own with power 4 or greater was exiled until the sacrificed creature left the battlefield, that creature will return to the battlefield before the check, so it will count. However, any abilities that trigger on the sacrificed creature’s death won’t have resolved yet when you check what creatures you control.

You gain only 4 life, even if you control more than one creature with power 4 or greater.

You can still gain 4 life even if the target opponent can’t sacrifice a creature.