Have you tried reversing while trying to disengage? Another way (if you have an FEL) is to just raise the front wheels off the ground and then try to disengage the FWD. You could also gently rock the HST pedal back and forth but not enough to spin the rear tires.
It should disengage when reversing because the fronts turn a little faster than the rears, there is always as bit of tension on them. Reversing removes this tension, but if you reverse too much without trying to disengage, you will put reverse tension on the front so keep tension on the 4 WD lever when doing any of the above.