I think this is probably something you'll need to play around with to get it working how you want it to.

What I'd suggest is the following:

ToonA -
/stopmacro [mod:ctrl]
/cast [mod:shift,help][target=mouseover,help][target=player] Healing Wave (leave out the mouseover if you want, that just gives you the option of healing someone other than your target from your main and you could put the mouseover bit before the [mod:shift] part if you want to be able to heal one target on toonb and one on toon a)

ToonB -
/stopmacro [mod:ctrl]
/cast [mod:shift,target=ToonA-target,help][help][target=player] Healing Wave

ToonC and ToonD -
/stopmacro [mod:shift]
/cast [mod:ctrl,target=ToonA-target,help][help][target=player] Healing Wave

I think (but am not certain) that if ToonA doesn't have a target then the macro will still continue to trying to cast on the toons target (if there is one) or themselves. Which means you could heal just 1+2 or just 3+4, potentially wastefull still, but not as much as everyone healing themselves everytime you press the key. If for some reason the macro doesn'tfall through you'll need to add in [mod:shift,help][mod:shift,target=player] after the [mod:shift,target=ToonA-target,help] part.

Along with these macros it would be a good idea to have a macro to select a target on each toon and one to clear a target on each toon. Possibly you could get this on pone button by using modifiers, but I'm not sure which way it would be most sensible to set that up. Also if you can append assisting/clearing target with a condition then it makes it somewhat easier. ToonA won't need a macro, as you can just select or deselect their target.

e.g.
ToonB -
/assist ToonA [mod:alt]

ToonC -
/assist ToonA [mod:shift]

And the same for cleartarget. If you can't add conditions to those then you'll need something more complicated, e.g.

ToonB -
/stopmacro [mod:shift][mod:ctrl]
/assist ToonA

ToonC -
/stopmacro [mod:alt][mod:ctrl]
/assist ToonA

This also has the down side that you can't select the same target on more than one alt at a time, whereas you could with the above method by holding shift and alt and pressing the key.