I am not absolutely sure, but hydroxylation at "benzoic ring" is better for scavenging quicker radicals ('antioxidant potential') than hydroxyls in the "stilbenic" ring. This would mean 2' is quicker reacting than 4' > 2 > 4. However, in terms of stability of the radicals formed, this can be different. Here, it would suggest that 4 is more stable than 2 > 2' > 4'