A Fable 4 announcement (or Fable IV, I guess) is likely on the way, but the game's appearance on Microsoft's Mixer streaming website shouldn't be taken as confirmation. Following the discovery of a listing for the game on Mixer, which was originally shared by a Reddit user, many have concluded that Microsoft has slipped up by adding it to its database too early.
But the Mixer database actually draws from player.me, an online tool for streamers with a largely crowdsourced database. At the time of writing, Fable IV has four 'players' on player.me, and the listing also appears on Twitch.
The player.me entry lists Lionhead Studios Ltd. as its developer: likely not a mistake Microsoft itself would make, since Microsoft closed that studio in 2016.
While this rumour probably doesn't mean much, it wouldn't be a huge surprise if Fable 4 were announced at E3: there are strong rumours pointing to Forza studio Playground Games working on a revival for the series. We already know that the studio is definitely working on a non-racing, open world game, with the working title Project 2.
It's been nearly ten years since Fable 3, and a lot has changed since: its original creator, Lionhead Studios, has closed, taking with it the in-development MOBA Fable Legends. That wasn't the death of the series, though, because a free-to-play card game called Fable Fortune released in 2018, though it didn't exactly set the world ablaze. A new Fable game would probably need to work overtime to make a splash in 2019.
In any case, if a Fable 4 announcement is imminent it'll likely take place during Microsoft's E3 2019 press conference. Here is the full E3 2019 schedule.