You are assuming that full frame lenses are better than other lenses.
I think that is not correct at all, there are wonderful smaller frame lenses and there aslo not so great full frame lenses.
Sure, full frame lenses can support higher resolutions, so yes if you absolutely need to make that 36 Megapixel movie go ahead and use a full frame lens but for 4K I think it is absolutely unnecessary.
Nooooooo, please not again
Ok, I will try to describe it again.. It is about lens selection!! I have never said that FF lenses are better, I am not saying FF is better, I am not convincing anyone, I don't say brand X is better than brand Y, I am not saying anything about DOF, I am just saying that there is way better lens selection when you are shooting FF.
So, again, lens selection. When I say selection, I mean number of different focal lengths, apertures, stabilized, fast etc.