It seems strange that they did an update of the K4 before the K5 (original Kindle Touch).
My only guess is the K4 was sold for almost two years - there might be a lot more of them still in-use.
But any how - - -
I expect the K5 to get an update next.
The 5.6.x firmware already supports the IR touch scanner (KT-2) and the older SoC (PW-1).
Ref:
https://www.mobileread.com/forums/sho...d.php?t=262279
Which I will amend once the 5.6.5 update releases seem to settle out.