"Ether way, one z4k can only run two conventional tracks, just the same as a single TIU with the WIU & premium App."
Apples & Oranges. The first half of the sentence is true, only if you have a remote or Z4k controller, but the remote w/ Z4k receiver, unlike the WIU, can control voltage output of 4 TIU channels and the WIU, can only control voltage on two. Go the WIU route, and you need 2 TIUs.
This discussion may be academic, since Z4k receivers are scarce.