All ST4 does is supply an earth pulse on either Ra+. Ra-..etc etc.
In the early days...this "earth" switched a relay coil which in turn supplied pwr to drive a motor etc.
Generally nowadays it is used to switch a
opto diode or even the emitter of an NPN transistor..or base of a PNP transistor.
In this pic the ST4 command (from Rj11 pin 6..from the autoguider) is an earth pulse to a opto diode...that drives a phototransistor.
In other words..the autoguider outputs an earth pulse (as appropriate commanded by guide software, either Ra+ or so on) which is used to command a drive motor system to move.
Hope this helps.