4 magic missiles at 1d4+1 is 5 x 4 for 20 damage.
His empowered magic missiles are four at 1d4+1 and 1.5 the rolled amount-- 1 is +0, 2 is +1, 3 is +1, 4 is +2.
For example: +0, +0, +1, +1 for 13 damage.
The Splatter Man is a 8th level caster, this means that every time he casts magic missile, it creates 4 missiles, each doing 1d4+1 damage.
The maximized missiles do 5 damage each. If a character is hit by more than one missile than the damage just adds on(5,10,15,20). Remember that the Splatter Man can easily take out a player if he focused all his missiles on one character, so don't do it, spread em out so the party has a chance. They will most likely be hurt from the collapsing room, so multiple missiles could start dropping people in this fight before you want them too.
The Empowered magic missiles add half the die roll to the total damage(minimum zero). The die roll remains d4+1 but the total damage is as follows
ROLL
1 does 2 damage
2 does 3 damage
3 does 5 damage
4 does 7 damage
The Splatter Man still shoots four missiles per casting of the spell even if the spell is empowered or maximized.
Be sure to keep in mind that a Shield spell negates magic missiles, so if one of your players has that up, they are immune to the damage. If you feel like your party is going to have a lot of trouble with this boss you may consider putting a shield potion in one of the item caches.
Remember that The Splatter Man can have a LOT of fun with the amount of missiles he is throwing around, but you really want to have the players feel more like they are on the ropes than caught in something hopeless. I suggest using the empowered and lower level ones for damaging players throughout, and use the maximized ones negate any healing that gets done. Also remember to keep his ghostly touch as a last resort for him, I personally played him as very proud of his magic and not wanting to give into physical confrontation as he believed he should be able to destroy these pests using just his spell casting. That touch can easily destroy a party if he just starts using it out of the gate, so only break it out once The Splatter Man gets desperate.
Anyways,have fun with this battle, it has already become one of my table's favorite bosses.