As of 8am UK this morning, Fortnite has gone offline for maintenance, and we don't know how long it will be offline for.
Brace for Impact! Downtime has started for the Season 4 update.
— Fortnite (@FortniteGame) May 1, 2018
Fortnite maintenance has in the past lasted anywhere from 90 minutes up to six hours, with larger updates generally meaning longer offline time. The current batch of maintenance is adding Season 4's new content to Fortnite, and so might end up being on the longer end of their updates.
When the game comes back online, we're expecting comets to have hit land, possibly destroying the Tilted Towers, as well as the introduction of new skins, possibly themed around alien superheroes.
We will update you all as soon as the update is live and we've seen what it brings with it.