I understand what you're wanting to do. I am not sure how it's possible with just one macro though.

I currently have Focus #1 - Focus #4 keys for my quad teams. When I want to be able to use 1 spell on 4 different targets with one macro button here's what I do.
I'm using my 4 hunters as an example. Each of them can cast Intimidation, a stunning attack from their pets, but I like to spread it out to control the caster mobs in a pack of mobs I attack.

I use Foucs #1 nearly 99% of the time. I have the following line in my macro setup:
/castsequence reset=combat Intmidation,,, (for main)
/castsequence reset=combat ,Intimidation,, (for #2) and etc, etc.

Then I have my #1 just target the random mobs and hit the keys accordingly.



Sorry if that's not helpful