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.
Source: Unleashed FAQ (2010-05-13), PUI Rules Team (2010-05-13)

Relevant cards

Note: All applicable cards may not be listed below.
BW2_26
HGSS2_62
PL1_SH5