I find it difficult to release 4 wheel drive with your thumb into the throttle. However, it is easier to let off, switch, then back into it. If it does not disengage, there is a problem in the solenoid, or wiriing/switch
If you are in 2wd and switch to 4x4, same thing, just let off the throttle, for a second and right back into it, you should be in 4x4 now.
That is why they put the switch by the throttle, so you would release the throttle, switch to 4x4 or 4x2 and then back on the throttle.
You DO NOT have to stop the machine to switch in or out of 4x4.
Engine brking in 4x4 mode is better on the newer models, but to enjoy it you must learn how to use it. Try going down some moderate inclines in 4x4, esp 1st gear and no brakes and watch how the wheel spin will disengage the engine braking.If the wheels do slip and it disengages, use the brakes to get your speed back down below 10MPH/15KMH and it should re-engage. In 2wd it is useless as the rear wheels will easily lock up. I believe engine braking was meant for 4x4 mode.