So I started a thread on the RetroPie forum but after yet more investigation is seems to be something physical - but I can’t see what :S
RPi3 + Picade HAT + Picade 8" = WORKING
RPi4 + Picade HAT = WORKING
RPi4 + Picade HAT + Picade 8" = WiFi won’t obtain IP address, but scan and signal strength return OK
Now, I’ve also tried lots of different configurations. I have 3 images I’ve used - Vanilla Buster Lite, PxlTbl (a working image from a very similar project I have, laser-cut case, 7" screen, pimoroni audio hat). I have an RPi3 and 2 RPi4s.
All the Pis work with all the other images and I’ve not got/had any issues with WiFi. I’ve rebooted all my network equipment, it’s quite a complicated setup but it’s all DrayTek business class stuff (I’m a DrayTek authorised reseller).
If I put the Picade hat on the RPi4 and leave on my desk - it works. If I assemble everything, just can’t get WiFi to work well enough to obtain an IP via DHCP. Assigning a fixed IP doesn’t solve the problem, I still can’t ping or send traffic. If I rebuild it with a RasPi3 - everything works just fine.