Bethesda: "Fallout 4 before 2018"
Fallout 3's radiation sickness hasn't even begun to spread and people are already talking about another sequel.
We like to keep people waiting. Three years seems to be right - Bethesda
So I guess Fallout 4 will be out in 2012. So for now, we'll just have to enjoy Fallout 3 for 3 years until Bethesda is finished making a sequel to the awesome series.