If you do the math and read how Bethesda development works, then you know Fallout 5 won't happen until the half or end of the decade.
Here's how they do it: Its actually pretty smart and efficient. You see, most devs have the concept and devs working at the same time, and then the concept guys get fired and hired again for the next game. Not Bethesda. Devs work on the current game, the Concept guys are already thinking up the next game, making concept art, designing, etc. When the current game finishes up (AKA last DLC done), the devs go to the next project and the concept guys to the project after this one.
This way, both the devs and concept guys are always working. Its very smart.
For example, the Devs are finishing Starfield right now. That means the Concept team is working on TES VI right now. When Starfield releases the last DLC, the Devs will work on TES VI and the Concept guys will work on Fallout 5.
Bethesda games are out every 3-5 years. Starfield will be out in 2021 for sure. Then an year or two of DLC. Then they start working on the next game.
So...
Starfield - 2021 and 2022
TES VI - Development time between 3-5 years. Which means launch at 2025-27. Maybe 2024.
Fallout 5 - In 2030?!
One flaw with my reasoning is that I can't believe they will let mainline Fallout titles sit on a shelf for literally fifteen years and pretty much skip the current gen. Fallout 3 and 4 have seven years of difference, and I suspect Fallout New Vegas is a thing because Bethesda wanted to keep the franchise fresh in people's minds during that interval.
I did hear they expanded and hired more people some time back. So they may have extra teams or something. TES VI may be in parallel development alongside Starfield. I think that's likely to be the case. Which might mean that Fallout 5 might be out in 2023-2025.
And I would not be a surprised if we have an spin-off, New Vegas-style, to have a new-gen Fallout.