Making a combo map with 3 RGB channels is a really good idea, however, putting a 4th in the AO channel doubles the compressed texture memory. Usually you can find at least one map that works fine with less resolution than the others, so, If you size down that one (for instance, 1024x1024 to 512x512) and save it as a second…
I thought about it a bit, and I don't recall ever actually putting a 4th channel into my combo maps. It's pretty much my standard procedure when I'm building textures to output from substance for UE4 that I add an extra output for my combo map.