Orlybus stops at terminals 1/2/3 as well as 4. If you travel all the way to 4 then the buses will already be packed full of people and you will find it difficult to find space.
Thank you for that update! I've updated the article with your information.
This was an interesting (odd?) decision by the managers of the Orlybus route to stop passenger pickup at Orly 1-2-3 in 2019 and to force everyone to board at Orly 4 as noted by other locals and myself the last time I was at that airport. Apparently this decision has now been reversed. So, thank you for the correction.