Can I install my new small under-counter Bosch ES4 four-gallon electric hot water tank on its side rather than vertically? PR valve would be on the side instead of on top. I’ve got a clearance problem. Thank you!
No. The installation manual for the ES 2.5, 4 and 8 says "Horizontal installation - ES8 Model Only". So no, you can't do that with the ES4.
Nope, and there's a Technology Connections for this.
The whole system is built around gravity, that's how it separates the heated water from the cold. If you misorient it, that won't work, and water temperature will be spotty.
The wall mounting plate holds the water heater with the two metal protrusions which go into corresponding holes in the back. Since the holes are only in one positions, there's no way to change the mounting.
Even if you managed to somehow mount it, as the others said, it still wouldn't work. The hot water outlet needs to be on top and the cold water intake needs to be on the bottom. This means the vertical model has an internal pipe to send cold water to the bottom, while th horizontal doesn't, hot and cold water fittings are simply spaced more.