The surplus actuator looks good! That's what the E-stop company is using, or something similar. They just have a limiting circuit. The "microcomputer" is probably a BASIC Stamp or Arduino board with some type of strain gauge -- probably measures current draw of the actuator. Simple -- motor starts to strain and draw more amps, computer cuts it off. Not sure if that's how they do it, but has to be something like that. They might have a more complicated and costly way to do it.
With a 4" stroke actuator you would need something to prevent pulling too hard and frying the actuator or wiring to it, or pulling something loose. A simple limit switch would work, but would need adjusting on occasion. I know the brake cable needs some travel, but 4" sounds excessive. I don't think the cable actually pulls but maybe an inch. The pedal or pull handle travels more, but they are made to have more leverage so they don't require a lot of effort to set the brake. There are several 1.5"-2.25" actuators on the surplus site (Surplus Center
) that would work and are pretty cheap. A few are electric seat motors. No pull rating, but they move my *** and seat along just fine, I'm sure they would pull a park brake cable hard enough! I like the 1.5" 250 lb rated actuator for this application, if a 1.5" pull is enough.