Review on Keyestudio 5V DCAC 4-Channel Relay Hat Shield Module Expansion Board for Raspberry Pi 4, A+, B+, Pi 2 Model B, Pi 3 Model B by Manuel Brown

Pros: * decent price * feature seems to work as advertised. The documentation is only useful if you use WiringPi.Cons: * The contact header is not passed through. I ended up adding another pass-through connector at the bottom for some distance and added longer threaded posts to compensate. * PCB traces for relay contacts are visible on the underside of the PCB; This is not very safe if you want to use the mains voltage control board (120VAC)! I added a layer of Kapton tape to the bottom to prevent accidental shorting and "smoke blowing". *Relay J2 uses GPIO 4, which conflicts with the default 1-wire functionality on the RPi. It would be nice if there was some kind of configurability with jumpers etc. It would also be nice if it was mentioned in the wiki documentation. Overall I'm happy with these boards. It would be nice if the vendor included the schema in the documentation, but I understand that this leads to a simple design.