In the process of. Though it's a lot older than 4 years... my first notes/ideas on it were a decade ago.
What's funny is other than a few of the characters being mostly the same, and the setting being the same, the first version was very different from the original notes, and the current version is even farther off. But this round I have added roughly 40k words so far, deleted or moved entire sections, and just overall, it's a hugely better story for it, even without the improvements in my writing.
In some ways it's harder to go back and fix the story than just start a complete rewrite from the ground up. I find myself trying to put sections in from the previous versions that turned out well, but no longer fit with the story flow, so you have to be willing to delete enmasse, which can be tough when you're emotionally invested in a story you spent so much time working on.
Not too mention the horror of rereading your old work and realizing just how bad it was.