Doesn't have to be a solo session. If you have the right group for it (big IF there) you can jump back and forth between the individual characters, essentially running four solo sessions in parallel. This relies heavily on your players being the kind of people who are invested in the action even when their character isn't present, but it can be done.

That said, I think for the most part the "Solo movie" should really be a character's backstory. This is why I don't like D&D, or at least the D&D presumption of starting at level 1. It leaves no room for characters to have an interesting history if they're basically at the level where the average house-cat is a threat. If I run D&D, I start people off at somewhere around level 5 - 10. Give them enough ability that they can actually have done some interesting things already. Get the solo movie out of the way before the game even starts.