Can we get the Navigo Decouverte card from one of the M4 stations (Porte d'Orléans)?
Navigo cards can be bought from ticket windows along Metro line 4 in Paris (or any Metro/RER/Transilien ticket window), but the caveat is that any single ticket window may have run out of blank cards. In that case you'll need to visit another station / ticket window to try your luck.
The major train stations (Gare du Nord, Gare Saint Lazare, etc.) Transilien ticket windows (the "brand" for their commuter trains) often have a lot more Navigo cards to distribute if you're having troubles getting a new Navigo card.