Fortnite Season 3 feels like it just started not long ago, and already it's speeding towards its conclusion that should come before the end of the month. The end dates for the games' seasonal battle passes typically give away the date for when the current season will end and the next will begin, though as we've seen numerous times in the past, there's always a chance the current season will be extended for any number of reasons. Assuming that doesn't happen, the current season should transition into Season 4 in just over a week.
According to Epic Games' battle pass information for Season 3, the current season is supposed to end on August 27th. Fortnite seasons typically start right away after one ends, so it's assumed that Season 4 would then immediately start afterwards on the same day, or if not then, perhaps August 28th. That new season means another theme, another battle pass, and another set of seasonal changes and features to look forward to.
Of course, there are any number of reasons why the current season could be extended which would at the same time delay Season 4. With the ongoing coronavirus pandemic and everyone having to adjust their working conditions accordingly, we've seen no shortage of delays and reworked plans for seasons, new games, and everything else. That means the next season could very well be delayed for a while until it's ready, though if that's the case, it'll hopefully not be delayed as long as past seasons have been.
There's also another matter at play here that hasn't been going on in past seasons and could affect the start date of Season 4. In a very publicized and dramatic battle of big brands, Fortnite was removed from the App Store and the Google Play Store and is no longer available on mobile devices. Legal complaints followed the actions as both Apple and Epic Games stood firm on their positions and outlined the consequences of each other's actions with neither willing to budge so far.
Matters like this probably won't be resolved within the week which begs the question of what will happen to Fortnite's mobile community if the season starts on the planned date. Those players would be left behind since they wouldn't be able to receive the seasonal update for the game and would therefore begin the season later. That would mean working out logistics of those players not having enough time to work through the battle pass, missing out on seasonal events that non-mobile players were present for, and throwing off the synchronization of Fortnite's updates. It's not unreasonable then to think that the new season could be delayed for those reasons to make sure every platform is still aligned with the others.
Whatever is to happen with the current season and the next, Fortnite always has a big end-of-season event to close out the current season. We'll either see that teased soon to indicate the end is coming, or we'll see communications from Epic Games to say the season's been extended.