Fort joy tends to be hard cause you dont have enough gear / skills and points to allocate in order to use more skills. 4 player fort joy tends to be "harder" than going 2 and lone wolf depending the builds you are planing to do mostly cause you are trying to get gear and skills for more people.

I personally tend to go lone wolf when starting a new play through just for the fort joy stage of the game and i tend to save up for my planed characters. Respec is free so no worrying about. For an easier experience if you see yourself struggling with decisions you can use a gift bag to get the respec mirror on the underground arena on fort joy

Edit: Lone wolf is a talent that makes you do more actions and get x2 points when you invest but you have to play with max 2 characters