By the way, some dogs are naturally attuned to territorial boundaries, others...not so much,
and there are other ways to teach boundaries than with a wireless fence. Even a wireless fence, should you be dead set on using one anyway, requires a good amount of training that may be more difficult to get across to an adventuresome 4 month old Yorkie puppy than putting up a simple physical barrier. A wireless fence is not as simple to use properly as clipping on a collar and letting the dog outside into the 'protected' area. Perhaps you might want to see if you can download an instruction manual for your chosen wireless fence to see their recommendations.
A chain lock and some removable velcro'd screening across the opening at the bottom of the 'offending' door may work better to keep both small child and Yorkie puppy in the house and out of danger better than attempting to teach wireless fence boundaries in time to avoid a more serious mishap. Please do understand that there are many alternatives.