I'm now continuing the design of a PCB (Pi Hat) that I started with a RPi 2 Model B.
Will a RPi4 accept this hat?
TL;DR: Does a RPi4 accept a Pi Hat designed for RPi 2 B? Or is there a break in the compatibility?
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The official design specification for a Raspberry Pi HAT hasn't changed since 2014 when the current form-factor was launched with the RPiB+. There's no hardware reason why a HAT for a RPi2B won't run on a RPi4B.
The thing that may need to change is the software driver as 32-bit Buster is a bit different to Jessie. The RPF/RP(T)Ltd. folks have just launched a 64-bit kernel for end-user testing.