This might be a silly question but,... did you measure 4.25V with the charge pump installed or out of circuit?
I've finished mine today, and the trimmers don't have the range to set the drain voltage that low. I've measured the voltage between drain and ground. At the lowest setting of the trimmers the voltage at drain was 10V if I remember correctly.
I took the charge pump out and checked the range of the trimmers again. Now I was able to set the bias to 4.25V. After installing the charge pump again the bias settled at 11.9V.
edit: After reading through some other
thread it looks like my JFETs are fake or otherwise inadequate. I had a couple of others and those get in the 4.25V range. Guess I'll have to wait till Mouser or other reputable sources have them in stock again.