Set effects

If the set effect for 4 pieces is X and the set effect for 3 pieces is Y, do I get X when wearing 4 pieces, or X and Y? I've put off actually asking this for ages because I kept thinking I'd figure it out myself, but last I tried -- keeping careful watch on my stats -- I gained an extra ~600 HP after equipping a fourth piece. ....despite the fourth piece -- and the four-piece effect -- having nothing of the sort as a bonus.

@_@

X and Y.
For example, if you have all 6 pieces of an empress set, you get all the listed effects, not just the last part (30 weapon att, 30% boss).