Is this a win or loss?
You won. The rules state that in order to win, you must discover the cure to all 4 diseases. The moment you discover the 4th cure, the game ends and you win.

From the rules:


  The players win as soon as cures to all 4 diseases are discovered.


And:


  The players do not have to eradicate all 4 diseases to win;
  just cure them. Once all diseases are cured, the game ends and players
  win immediately, no matter how many cubes are on the board.


(Emphasis mine)