Traveling with 4 flashes.... I don't see, besides the price. Traveling light is the best thing and with the 2x Z330 I have lost that advantage. The bad thing is that selling the Z330 you lose a lot of money.
Obviously the Z330 with its greater power, will illuminate the reef better, but I think the S220 can be perfectly valid for it.
The S220 has significant improvements over the S2000, such as the standard dome, is equipped with a bayonet mount to use various dome filters or improved button size or pre-flash button placement, operates on four AA batteries and double o-rings that significantly reduce flooding risk.
It has the best of both worlds S-2000 and Z330 Type 2.