Turning on 4 zones + 1 master at the same time? Hmm, this is a lot of current. In that case, I would recommend you to power RPi separately using a 5V USB adapter and microUSB cable, while 24VAC will only need to provide current to the valves. If you do so, I recommend you to remove the PTC fuse (which looks like this: http://www.sellifuse.com/pptc/imges/2016_big.jpg) to avoid two regulated power sources competing with each other.
As I said in previous posts, I also think RPi Zero and A+ are the most suitable for OSPi as they are cheap, power efficient, and pretty sufficient to handle the computation need for OpenSprinkler firmware.