I am going to Loire Valley next week. I will be taking a TGV. My ticket says I have to switch from Paris Nord to Paris Montparnasse 1 et 2. I have 45 min to go from Nord to Montparnasse. Can someone please tell me what's the best way to do it? I have read on some forum that I need to take metro line #4. Can someone please explain? Also, do I need to pay for the metro or is it included in my TGV ticket?

I would appreciate your advise. Thank you.