Just came across this looking for the same answer and have to agree with what's been said.
If you can only recruit four undead why are there 10 figures supplied?
Hades allows you to purchase 4 additional undead so you can recruit a total of 5 with him. There are 10 miniatures to cover any eventuality (I.e. 5 troops, 5 fleets or a combination of the 2).