If Spearow has less than 4 damage counters can I use its "Roost" attack?
Yes. You do as much as you can to perform the effect of the attack; in this case remove all the damage counters.
Roost



x2
-20
RoostHeal 40 damage from this Pokémon. This Pokémon can't retreat during your next turn.

Rain SplashIf Spearow has less than 4 damage counters can I use its "Roost" attack?
Yes. You do as much as you can to perform the effect of the attack; in this case remove all the damage counters.