Id try running all 4 of 2 channels because you can.
Pretty good chance you'll need to turn them down alot so the overheads are "barely there".
Then when things get wild on screen it won't be distracting but rather will scare the crap out of you and anyone listening.
If you do that, you would only be setting up your receiver as 5.x.2 and not 5.x.4.
Not a big deal if your ceiling speakers are close together and if the source material coding doesn't take full advantage of ATMOS (of which very little material does at the moment).
However, if you want true 5.x.4, you need to get an additional 2-ch amp. Seeing as it is for ATMOS ceiling, it need not be too powerful.
