Bethesda will be revealing their next game at 3pm GMT tomorrow, posting this impossible-to-decrypt image:
Bethesda's previous solo game was The Elder Scrolls V: Skyrim, released to immense success in late 2011. Although their partner studio Zenimax subsequently released The Elder Scrolls Online, the core Bethesda studio hasn't let much slip about their next single-player game in the past four years. Fans and media alike have been anticipating Fallout 4, based on the immense success of both Fallout 3 (2008) and Fallout: New Vegas (2010), the latter co-produced with Obsidian Entertainment.
The teaser image suggests that this speculation is correct and that Bethesda will almost certainly announce Fallout 4 tomorrow, possibly with a CG video produced by Del Toro Studios. A longer presentation, rumoured to focus on actual gameplay, will be made at the E3 games expo later this month.
It has been heavily rumoured that Fallout 4 will be set in Boston and focus on the Institute (probably descended from the Massachusetts Institute of Technology), an area of advanced scientific study responsible for the androids and some of the other crazy backstory elements in Fallout 3. With Bethesda also saying they wanted to focus on the east coast and leave the Mid-West and west coast areas to other developers (probably Bethesda), this makes the suggestion of Boston as a setting more likely. The other crowd-pleasing choice, New York, would likely require a more advanced engine to depict the massive city in more convincing detail. Bethesda indicated a while ago that their next game would still be using Skyrim's Creation Engine (itself only a modestly-weaked version of the old GameBryo Engine used to power everything from Morrowind onwards) instead.
We should learn more tomorrow.