Well, now that you mention it, the logic board on the iPad Pro is also a narrow rectangle. But I really doubt they use the exact same logic board design for both devices…The MacBook Airs use a “computer on stick” logic board (similar but not identical to the approach used in the current iMac). Is that the board approach used in the M4 iPad Pro?
The moment to up the RAM was with the new M4 iPad Pro. I wonder if they will unlock the remaining 4GB in the future, via software update.