first 4 z steppers is not optimal. 3 is better.
see
https://drmrehorst.blogspot.com/2017/07/3-point-print-bed-leveling-vs-4-point.html
the maestro is discontinued because a new version is coming out soon.
you can still get the extension boards like here https://www.filafarm.de/collections/duet3d/products/duet-2-maestro-dual-stepper-driver-expansion-module
i would recommend to get the duet 3 because its more future proof.
if you go with the duet 3 and 3 z steppers you could go with the toolboard extension
https://www.duet3d.com/Duet3Toolboard1LC
if you want 4 z steppers you would need the 3hc
https://www.duet3d.com/Duet3Expansion3HC