Season 4 does not get the correct season poster. Somehow Plex doesn’t see it. It sees another poster which is not on TMDB at all, and the selection in Plex web is empty (despite Plex fetching a poster). Maybe some linkage is wrong?
Plex uses image data from a different provider than it used to. They kept getting copyright strikes from some movie companies over hosting and providing the poster to certain movies (The Good, the Bad, and the Ugly). As a result, a lot of posters you see on various metadata sites may not be automatically downloaded.
You can manually pull the poster from TMDB and upload it to your own server though. It shouldn’t be too hard to do this for Sliders.
I had a look at this last week when I seen your post and nothing obvious stood out to me. Its also an issue on the plex cloud at https://watch.plex.tv/en-GB/show/sliders where the season 4 poster is defaulting back to the show level poster.
My guess this is a back-end metadata problem the plex team themselves will need to fix.