The good news for fans of the Fallout franchise is that the fifth mainline entry in the series is officially going to happen. The bad news is that it will be a long, long while until we get our hands on it. Or even see anything of the actual game, for that matter.
In an interview with IGN's Ryan Mccaffrey, Bethesda Game Studios director Todd Howard confirmed that Fallout 5 will eventually happen, saying "Yes, Elder Scrolls 6 is in pre-production, and, you know, we’re going to be doing Fallout 5 after that, so our slate’s pretty full going forward for a while. We have some other projects that we look at from time to time as well."
Howard has previously stated that Bethesda wanted to complete Starfield before going all-in on the next Elder Scrolls title. With Starfield set to release sometime in 2023, and given the amount of time it takes to bring to fruition the kind of large-scale projects Bethesda is known for, it is likely to be years until we see TES 6 – likely sometime in the latter half of the decade. Which means Fallout 5 won't arrive until somewhere around the end of the 2020's judging from Bethesda's previous track record, if we want to be realistic here.
Other than that, there is no info on the game whatsoever, although Howard mentioned last year that the company has a "one-pager" outline document on the game. At least we have Fallout 76 to keep us company in the meantime.