Both Fallout and Elder Scrolls are two of the biggest gaming franchise in history and they are both owned by Bethesda. If the law of chronology is to be applied, then it is only right if Fallout 4 is to get released next by the game developer.
However, things are different for Bethesda as plenty of leaks and rumours are suggesting that Elder Scrolls 6 will get released ahead of Fallout 4. The word is that Bethesda has decided to produce Elder Scrolls 6 first because the Elder Scrolls franchise is a lot more popular than the Fallout franchise.
If the rumours are right, then it will be a much longer wait for fans of the Fallout franchise. Also, seeing Elder Scrolls 6 getting released ahead of Fallout 4 will definitely upset the Fallout fans in every way. Heck, even Elder Scrolls fans are agreeing that Fallout 4 deserves to come first.
Game analysts are indicating that the decision was partly due to the rivalry between Bethesda and Bioware. The latter had just launched Dragon Age Inquisition and Bethesda aims to challenge it with Elder Scrolls VI: Argonia. When Bioware releases Mass Effect 4, only then will Bethesda start working on Fallout 4.