Makes sense. Can't match two of the same tiers when both have different values.
Question then: S4 bonuses are same for Retri and MM hunts like S3 bonuses. If I equip 2 pieces from S3 and 2 pieces from S4 will those 2-piece bonus stack twice or just once? Because if the bonuses are different (like Prot pala gets S1 bonus now) its normal to stack x2 2piece bonuses from 2 different tiers.