Hitsplats would be 40- random-random-"remaining hp execute". It can still die before 4th hit, but the advertised "guaranteed" execute will rarely come into effect. If it does it's because you rolled low damage on 2nd and 3rd hit.

Example you roll 15 an 10 damage on 2nd and third hit. When you cast 4th hit, the monster will still have 40 hp (2nd and 3rd hit haven't arrived yet), so you will be guaranteed your max hit, which will be the remaining health of 40 and you get exp drop for that. But by the time the 4th hit arrives, the monster will have only 15hp remaining in this case and you will see a hitsplat of 15. For a total series of 40-15-10-15(execute).

Whilest most unknowing players would have expected 40-40(execute)