> I don't really understand the purpose of RPi 4 and 5. They're not cheap compared to the price nor very powerful in any measure.

They are good for commercial installations like smart displays in stores (think big screens with menus behind fast food counters) and information kiosks. The extra HDMI port lets you drive two screens with one pi and the extra processing power keeps the UI smooth on high resolution. They also have hardware acceleration video decoding for shops wishing to play hi res promo videos and hobbyists building media terminals.

Cost is not a major concern here because the installation volume is low and there are far bigger expenses anyway. Just take a look at how much commercial displays are. The Pi company’s future supply guarantee is also nice because you know that within a given number of years if something breaks or you need another screen, you can just buy another identical pi and be done with it. Good luck sourcing a Chinese mini pc with compatible footprints, port orientations etc five years down the road.