I don't think I'm understanding why modifiers won't work. Are you already using up every single action button + every single modifier? If not there shouldn't be any issue with writing a modifier directly into your healing macro (unless there is something I am missing here...)
Shammy1 healing Macro:
/cast [nomod, target=shammy2] Heal spell
/cast [mod=shift, target=focustarget] damage spell
/cast [mod=ctrl, target=focustarget] damage spell
/cast [mod=alt, target=focustarget] damage spell

Shammy2 healing macro:
/cast [nomod, target=focustarget] damage spell
/cast [mod=shift, target=Shammy1] Heal spell
/cast [mod=ctrl, target=focustarget] damage spell
/cast [mod=alt, target=focustarget] damage spell

Shammy3 healing macro:
/cast [nomod, target=focustarget] damage spell
/cast [mod=shift, target=focustarget] damage spell
/cast [mod=ctrl, target=Shammy4] Heal Spell
/cast [mod=alt, target=focustarget] damage spell

Shammy4 healing macro:
/cast [nomod, target=focustarget] damage spell
/cast [mod=shift, target=focustarget] damage spell
/cast [mod=ctrl, target=focus target] damage spell
/cast [mod=alt, target=Shammy3] Heal spell

With this setup if you assigned this macro to Action Bar button 1, and you press [1] Shammy1 heals shammy2 and the rest dps, [Shift+1] Shammy2 heals Shammy1 and the rest dps, [ctrl+1] Shammy3 Heals Shammy4 and the rest dps, and [alt+1] Shammy4 heals Shammy3 and the rest dps.

This macro still only takes up one action button (the same amount as your current healing macro). You could also change it so that [nomod] always =dps on all shammies and the different mods = different heals, that way you would combine your dps spam, and healing macro into one button, if in fact space is limited.