As far as I know all Harmony models use a standard mini-USB connector, so any mini-USB cable will work. Many digital cameras, cellphones, and numerous other devices use this type of connector. If you don't have one already the cables should be widely available.
I don't think it's a 4 pin mini usb...perhaps a 5 pin mini usb (actually, counting the connectors in the port of the remote, there are 5 connectors)
so just make sure you get a 5 pin mini usb and it should work
Samsung LCD HDTV (LN52B750), Comcast X1 HD DVR Cable Box, Panasonic BluRay Player (DMP-BD55), Onkyo Receiver (TX-SR806), XBox 360, Nintendo Wii, SlingCatcher, and Apple TV (64GB 4th Gen.)....all with my Harmony Ultimate Home
Samsung LCD HDTV (LN40B640), Comcast X1 HD DVR, Panasonic Blu-Ray Player (DMP-BD60), SlingCatcher, and Apple TV (3rd Gen.) Monoprice HDMI Switch (HDX-501)....all with my Harmony 900 w/ 2 Blasters (Note: I am not affiliated with Logitech®, Inc.) Have you seen the FAQ Sticky? Useful LinksDefinition of Delays