Hello I just wanted to clear out the question that the Navigo Day Pass zone 1-4 covering the Orlybus also ? that is in the Zone 4. And also it is need to buy the navigo Easy Card 2€ and then load it with the Navigo Day pass Zone 1-4? Or you can buy the Navigo Day Pass directly without the Easy Card?
Orlybus is covered by Navigo Day pass 1-4 zone as Orly Airport is within Paris Transport zone 4.
Yes: you need to buy a physical card: Navigo Easy pass (cost 2€) or Navigo Decouverte card (5€) to buy the Navigo Day Pass and load it onto the card. You cannot buy Navigo Day Pass without a card.
Another option: you can buy a 1-4 zone Ticket Mobilis, paper multi-use day ticket for Paris. Ticket Mobilis has no plastic card / no additional fee and can be purchased from ticket vending machines. To buy Navigo Easy and Navigo Decouverte you must visit a ticket window or ticket office.
Ticket Mobilis is the same price as Navigo Day pass for the same zone coverage, but without the card fee. Years back the Ticket Mobilis did not include/cover CDG & ORY airports, but Paris airports are now included in Ticket Mobilis (since 2017).