The blocking restriction is applied only when blockers are declared. Once a creature equipped with Hedgewitch's Mask becomes blocked by a creature with power less than 4, increasing that creature's power won't cause the equipped creature to become unblocked.
Equipped creature gets +1/+1.
Equipped creature can't be blocked by creatures with power 4 or greater.
Equip (: Attach to target creature you control. Equip only as a sorcery.)
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