Gare du Nord to
Gare Montparnasse is definitely doable in 53 minutes, but 15 minutes is very optimistic. It'll take nearly 10 minutes to descend from the Eurostar and find your way to the RER B which is several levels down. Plus you'll have to purchase tickets for the RER B as well (you can use regular
Metro ticket t+ tickets), which can take anywhere from 2 minutes to 10 minutes depending on whether you can use a vending machine (if you have a smart-chip credit card or euro coins) or if you need to buy from a ticket booth. Then transfering at Denfert Rochereau to Metro Line 4 will take at least a few minutes given that you'll likely have to wait a minute or two for the next Metro train.
The route itself is fine, I have no complaints about that. And trains generally arrive & depart at very exact times so you should be OK to make this transfer, even though the timing is somewhat tight. As long as there are no unusual delays, you'll be fine.
Best of luck,
Ben