no this isnt unity, almost everything is handled differently.
current gen IOS does not handle 4k textures, they are reduced to 2k, regardless if its compressed or not. (afaict, various mobile devs I talked to)
you might be correct when the vertex displacement is set to such a texture, but generally internally all textures are made square, regardless of their usecase. (it wont mess up uv maps, ui layout etc) just how it works.
its best to try it I guess, only sure way to know for sure since occasionally there are even conflicting statements in the wiki.