So... after extensive testing...
Any 4 shroud accessories give a four piece set bonus to spell critical AND melee critical damage.
The amount of the bonus is determined by how many matching accessories you are wearing, ex. 3 dominion and 1 escalation would give the 4 piece set bonus, BUT the amount of the bonus would be determined as if you were wearing 3, due to only the 3 dominions matching.
With 3 tier 3 opposition and 1 tier 1 opposition you are getting 25% spell critical damage and 11% melee critical damage, along with 26% maximum hitpoints. With 3 Tier 3 opposition and 1 tier 3 escalation you are getting 23% spell critical damage and 10% melee critical damage, with no set bonus. With 3 tier 3 dominions and 1 tier 3 escalations you are getting 26% maximum SP with 23% spell critical damage and 11% melee critical damage.
Enjoy the info guys!