I own a Denon 4400 and the best thing about it, in my opinion, is sub-eq, that alone is worth purchasing an A/V receiver.
Atmos might be worth the time and investment if you have already moved to the 4K. It seems to me that Atmos is being included and primarily promoted as a 4K Blu Ray format. If you haven't moved to 4K (i.e. 4K TV, 4K/UHD Blu Ray player),like me, Atmos is scantly included on regular Blu Ray titles. I have a few titles myself, however, in my opinion, not really enough to justify the cost of the Atmos upgrade. I also have a few DTS-X titles, still not enough to justify the upgrade cost. Hopefully DTS-X can become standard for regular Blu Ray titles since DTS-HD is pretty much a standard on them.
I personally like to experiment, I would try it and see if you like, if not you could return your extra speakers. However, you don't want to do in ceiling speakers on an experimental basis. Anyway, the Denon 4400 is nice receiver, with exception of what I think is a little under powered, and would be a good receiver even without the Object based audio formats.