If the choice is between tank + 3 shaman + druid or tank + 4 shaman, I would go with tank + 4 shaman every time. Also I'm a die hard 4x elemental shaman fan, I will not respec a shaman resto.

With ebon plaguebringer as you pointed out earth and moon is pointless, and so you have to look at what a Druid would bring that a 4th shaman wouldn't bring. IMO the 4th shaman has more advantages - an extra healing stream or mana spring totem is invaluable, an extra magma totem for AoE, an extra grounding totem, and extra tremor totem for slightly less time between pulses, etc. Also, less complex macros because all 4 clones will be able to run the same cast sequences/spells/etc vs having to be more complex by using different macros for the druid.

This of course is just my opinion - I'm all for simplifying 5-boxing - I want it to feel fun and fluid, not like a job