Ok so it looks like I need to use either an optocoupler or a Relay Box with it if I'm using an ST4 style autoguider.
Originally I was informed (by someone that should know) that I wouldn't need an optocoupler. I do have a relay box so I'll give that a try next time.
What about if I'm using my laptop to connect to the Autoguider port? That's the part that's got me really confused. If I run to the GPUSB then I shouldn't need an additional optocoupler, it should (as I understand it) run straight from the adapter with an ST4 type cable.