I'd recommend starting with Fallout 3.
This will give you a good idea of how the various quest lines function, the leveling system works, and the overall atmosphere and pace of the Fallout world.
That way, once 4 arrives, you'll have some good ideas on how you want your character to develop. Plus, have much more appreciation of how amazing Fallout 4 really is.
Next stop, Boston..
You can have some ideas on you character development but Fallout 4 has changed it so that your character build greatly impacts the game more than previous Fallout games. Also, they have changed the way you build your character stats. But Fallout 3 still would be a great way to start your new Fallout adventure.