In a response to the number of questions Bethesda Games Studios has received about the end game structure of Fallout 4, they've taken to Twitter to provide a short and very meaty response. First up, unlike Fallout 3, the game won't end when the story concludes. Hooray! And secondly, there won't be a level cap. Double hooray! That means you could potentially roam the wastelands for as long as you want. How does forever sound?
The biggest change from Fallout 3 and New Vegas lies with the removal of a level-cap, which will be interesting to see how it gets implemented. Because naturally this could lead to balancing issues for any post-release content or planned expansions.