After all night trawling forum posts from 2009, I have finally given up looking for methods to add voltage control to the 4890 and just flashed the bugger!
Edit: Gone from a max core OC of 900Mhz to 1100Mhz and still climbing.
I was in two minds before stock landed with us. It would be nice getting a nice wod of boints but a waste of time with Ivy so close and then when Ivy drops, Kepler is so near too. Ivy+Kepler=Deadly. So I spent £500 on two more Acards instead