How to control 4 thermistors in a heated bed?
You can use the Klipper firmware which accepts an arbitrary number of heaters.

Then you can write a Klipper macro which takes one input and applies it to each heated bed (or not, as you prefer).

With Klipper you can also add another Arduino and connect it to klipper as well, in case you don't have enough pins on your board: klipper is not limited to controlling one single board per printer.

Check the docs.