Hi Rob,
The GB4000 is not the only pad device that generates a carrier. The F125, F165 and F170 have mulitple channels and one of them can be used as an RF carrier.
Also, the FDS signal processor can be used with Frex (running on your PC) to generate clean square waves. I've head of fantastic results using Frex.
And this device also generates an RF carrier (only up to 350kHz, but according to reports this is working very well, maybe as good as the GB4000).
I've measured the frequencies using Frex and they are well within 1Hz up into the kHz range (on several PCs). This is much better than 1% accuracy and therefor fine for Rifing.
So for those on a tight budget there are cheap, effective alternatives