So I am interested in buying a Holy Trainer v4 as my first chastity but I would like to know if it is possible to have a numbered tag lock used. That is not for me but for my key holder.
No that's not possible, altough you can buy this You can put the key in the cilinder and then put a numbered lock trough te holes, that way you can't use the key, simple as that It is also possible to make that tube yourself by drilling some holes in a pvc pipe, and cut open the upper part of the pvc pipe.