This is incorrect. Atmos will use whatever amount of speakers it needs to position the object where it needs to be. A front-to-back flyover would be interpreted as follows. It starts in the front speakers, then will rise upwards halfway in between the fronts and the tops equal volume (a phantom image), then fully to the top, and then half mixed between the rears and the tops equal volume (another phantom image), and then to the rears discretely. In my room, I can definitely hear a top front and a top rear phantom image between my front and rear speakers, and the tops.
The problem with using four overheads in a very small room is there is not sufficient distance between the speakers. They sound blended together when they should be discrete. It is not positioning, and it is not the dispersion pattern. They are just too close together, so they muddle and blend everything together.
Yes, i know that the object positioning that the decoder/processor does is trying to reconstruct the ideal positions and movements in the matrix with any number and any positions of the speakers, but as we know that our brain is actually creating the effect of recognition of sound sources in the space above, on the sides and behind, which are being received only with our two ears, via autonomous very fast processing algorithms, deciding upon the perception of space that is based mostly in the delays and reflections in the soundfield.
So i really don´t think that you can sucesfully create the same effect, as having the upper tops, because the main stereo pair is already recognised by the brain as being in front, and is continuously playing sounds that belong in the front part of the soundbed, if you get my meaning.
To be sure we should perhaps make an experiment in already properly set and working Atmos installation with four overheads.
I have perhaps every Atmos and True HD demo possible, but what i am missing is any demo disc, device or app, that would actually allow you to test in your installation how well is your system reproducing the mix. Do you know about such thing perhaps?
PS in very small rooms the thing that you are saying might be true, honestly i have never installed or heard Atmos in a "very small" room..
I am currently working on my smallest listening room right now (12 square meters, so very small and hopefully fitting your description), and i have already done the wiring for 5.1.4, so i will let you know soon