My prototype 7x14 lathe came with a 4 post toolchanger.
It it driven by a reversible AC motor. It rises up off the locking pin and start to rotate, The is a loud click about every tool position from a pawl (I guess). once it is past a tool position the reed switch is activated. The motor is then reversed. It rotates backwards until it hits the pawl then lowers onto the locking pin.
I'd advise against this type of changer for the following reasons.
1. Long tools such as boring bars are problematic as they hit the AC motor or possibly the work piece.
2. There is the possibility for swarf to get under the mechanism when it is raised and rotating. This will stop it seating properly.
A vertical rotating changer seems the best option.