is there a macro for this?

[Heenan]

1. the reset=6/target clause, how does "6" get computed here? Is that 6 seconds from when the macro was last executed? Or 6 seconds from the last combat etc?
2. Maybe what I need is reset=6/combat instead. I am still at work now so I can't really try this out, was just figuring out in my head.

The timer fires as soon as you activate the macro. If you have not completed the sequence in 6 seconds, then it resets back to the beginning. Since you're Disc I assume you have no Mind Blast/Holy Fire talents? So it would be best to set reset=10 for the CD of Holy Fire.

'Target' also forces a reset... I assumed you'd want to get a fresh Holy Fire dot on the next mob. Of course, if you're killing too fast this may be too much, and you'd want to modify further.

If you're disc/holy it might be better to do:
/castsequence reset=10/target Holy Fire, Smite, Smite, Smite, Smite

You'll always dot up a new target and then Smite away. If it's been 10s since you clicked the macro it'll reset back to Holy Fire.