If I remember correctly, you have the 4550 XLG, which has the same negative stage as the 4500 II. Let me say that the negative carrier frame is locked quite securely in place when you flip the lever. In fact, once you lock the negative carrier 'frame' in place, it does not move. Yes, you can move it if you push hard enough, but if you're doing that sort of movement to the enlarger, you've got other things to worry about.
That is the reason that I went to the system that I have and use. Unfortunately, I know of no way to remove the negative holder from the enlarger for mask changes without releasing the lock lever. If the frame that Lynn has developed is locked by the lock lever and the negative holder is removed and reinserted the very fact that is removable is indicative of greater the .005 available spacing since a "machinists press fit" is .005. The term "machinists press fit" means that with .005 a human being could not physically insert one piece into another without the use of mechanical aid in the form of a hydraulic or mechanical press. It is at this point when one would remove the negative holder to change masks that the negative holder registration would move. In moving the negative stage, it would throw the "whole shebang" out of registration. Even taking the fact that greater then .005 spacing exists (it must if one is physically inserting and removing the negative holder), when one enlarges that by a factor 800% (16X20 enlargement from 4X5 camera negative) the .005 becomes .040. This is a highly noticeable lack of registration. In fact, I imagine even a neophyte would notice that in a print.
My registration system while more expensive and I believe more precise is still relatively crude in comparison to another photographer here in Kansas(Charles Phillips) who does tri tone separation masking through a $61,000 enlarging system. This allows a selection of higher contrast filtration through the highlight and shadow regions of the paper (toe and shoulder of the papers characteristic curve) and another contrast filter selection for the midtones where tonal separation is normally greater. He has shared that even a very costly Condit system that was incorporated was prone to registration problems.
The potential for masking extends far beyond what Lynn Radeka has addressed. Lynn has taken from the earlier work of several photographers that were exploring masking. While beneficial and available to the masses it does not fully cover the potential of this process.