For a fan looking to binge the series now, the best advice is to ignore the "Season 4" label on streaming services. Treat War of Underworld as the second half of a single, massive season. By watching all consecutively, the viewer experiences the story as it was intended: as the most ambitious and cohesive narrative arc Sword Art Online has ever produced.
Here is the clear, spoiler-free breakdown of exactly how many episodes make up SAO Season 3 and how to watch them in the correct order. sword art online season 3 total episodes
(Episodes 25–47)