Bethesda has updated the Fallout website with a countdown that is pointing towards some sort of reveal for tomorrow. At the time of this posting, the timer is just under 18 hours remaining.

Fallout 4? It's probably Fallout 4. Maybe.

The page source also includes a bit of error text in case of error.
Access Error
PA system failure has occured.
Shutdown of the Masterbrain has been authorized and all sensitive materials have been removed for security purposes.
Please attempt re-access. Have a pleasant day.

Thank you, I will have a pleasant day.