Yeah, the average mean for a d4 hit die is 2.5, so 16*2.5-32=8.

It should have 8 hp. Of course, abberations should have d8 hit dice, with a mean of 4.5, bringing this critter's hp to a nice, round value of 40 (16*4.5-32=40), which a 12th-level party will have no trouble dishing out.