If you haven't hit the snowy part of the game yet or are still underwater, don't worry — things will get clearer before long, and there's plenty of gameplay left!
You might want to stop reading this thread until you finish the game though so you don't see spoilers.
The only thing that isn't really explained as far as I'm aware is where exactly the four year number comes from. I mean, it'd be extremely coincidental if everybody took exactly four years to get from one end of the Land of the Dead to the other.
The only conclusion you can really draw from that is that there's either some higher force which ensures nobody can get there in less than four years (fate?), or you simply won't be able to get through the gate to the 9th underworld until four years have passed.