I just tested it out with:

/cast [target=cloverine,nodead] lesser healing wave
/cast [target=clovine,nodead] lesser healing wave
/cast [target=clovianas,nodead] lesser healing wave
/cast [target=clovina,nodead] lesser healing wave

It works -- it goes down the list so each shaman targets the next one if that shaman is dead so it should always be casting a heal on someone every time regardless of who is dead as long as at least 1 is alive.

Shaman 1:

/cast [target=cloverine,nodead] lesser healing wave
/cast [target=clovine,nodead] lesser healing wave
/cast [target=clovianas,nodead] lesser healing wave
/cast [target=clovina,nodead] lesser healing wave

Shaman 2:
/cast [target=clovine,nodead] lesser healing wave
/cast [target=clovianas,nodead] lesser healing wave
/cast [target=clovina,nodead] lesser healing wave
/cast [target=cloverine,nodead] lesser healing wave

Shaman 3
/cast [target=clovianas,nodead] lesser healing wave
/cast [target=clovina,nodead] lesser healing wave
/cast [target=cloverine,nodead] lesser healing wave
/cast [target=clovine,nodead] lesser healing wave

Shaman 4
/cast [target=clovina,nodead] lesser healing wave
/cast [target=cloverine,nodead] lesser healing wave
/cast [target=clovine,nodead] lesser healing wave
/cast [target=clovianas,nodead] lesser healing wave