I don’t think you can do it directly on a windows box anyway.
(I master through Avid DS, not Media Composer, so I’ve never delt with this issue inside MC)
Can MC do a 4 channel output…yes, you can do a direct output.
But I also think Avid needs to be set as same as source for this tab to become active. So in your situation, you would need your avid timeline video to be prores in the first place. You could do a mixdown to prores, but you don’t have the Miraizon codec installed, so that isn’t really an option. You could try a direct out and then attempt Bouke’s idea. (I’m going to look into that one myself)
Personally I would call the client and see if they can take an DnxHD codec instead. I find a lot of people ask for ProRes “just because” It isn’t necessarily because it is the only file type their system will play. (doesn’t hurt to ask)
You could also ask a friend with a mac to convert your file to prores as well, but be sure the levels don’t get screwed up along the way.
Glenn