I have seen both ways, but never have heard of it needing a new order if you release one or two at a time. At my current facility we just release all 4 at once. Maybe your facility has a policy on it, or should specify if not. The issue I could see that would cause you to need a new order would be if you have to re-apply the arm and the leg if the patient "fails" the trial, which could be seen as re-initiating restraints.
Technically, if the patient is calm enough that you feel comfortable releasing two restraints, why not all four, since they would be able to remove the other two if they wanted.