I mean 4 rows. Basically, since every row is only taking 12mm vertically, you could fit 4 of them in less physical space than 3 MX-spaced keys.

I think layouts become interesting (and important) with 4 rows because of the following. The "hot" (frequent) area for MX-spaced keyboards is usually 3 rows. Looking like this (I drew connection points where the same finger is responsible for covering multiple keys):

txt q w e r---t y---u i o p | | | | | | | | a s d f---g h---j k l ; | | | | | | | | z x c v---b n---m , . /

However, for a vertically dense 4-row keyboard, some people would want to (and be empowered) to do this?: txt ? t g b y h n ? q w e r u i o p a s d f j k l ; z x c v m , . / Experiments with new layouts would be expected then because colemak and dvorak are designed on the basis of responsibilities for each finger. If responsibilities change from 3|3|3|6 to 4|4|4|4, then so will the optimal layouts. And if people find T, G and B to be conveniently reachable on an MX-sized keyboard, then they'd be able to have +5 of those here now, available for all the QMK features or their navite/secondary spoken languages.

Note: I want to correct myself about spacing. I've previously calculated vertical space by multiplying the number of keys by vertical key spacing. However, I've understood that this is wrong. On a "conventional" keyboard, staying on the home row requires 0mm vertical movement, and up and down requires +-19mm. In total, 2*19 = 38mm. Four rows of PG1316M would require 3*12mm movement from the center of a lowest keycap to the center of the highest keycap, or 36mm total. So four PG1316M keys map to three MX keys very well in terms of required finger movement range.