but why absolutely wanting to stay on the zoom x4?
This is what I don’t understand
in my opinion you take the problem upside down.
I guess that you say “zoom x4” maybe because you want to have 16th notes on each step…?
which is indeed the case with a lower number of 4 and a zoom x4 but it is no longer the case with the other lower numbers
in fact you should not worry about the zoom when you choose the length of your pattern and the number of steps
because the zoom does not change the length effectively (I mean, your pattern will always have the same duration whatever the zoom is)
it is the lower number that changes your resolution.
so if you want 15x (16th)notes per bar you just have to choose a TS of 15/16, and you stay on the zoom 1
this will be much less confusing if you have per example a pattern in 4/4 on another track with a zoom x4 (therefeore 16th note on each step)
because when you switch from one track to another you will have always the same note duration on each step (with polymeter option of course)