Im not saying it's an overpay, because it's not, but easy with the 4 WAR.
A lot of that is tied to defense, which is suspect to begin with. Maybin is kind of the prototype for why some people don't buy into WAR, because he was pretty much an average player last season, in large part thanks to his high SB totals and good SB%, because he isn't a great OBA guy and has no power.
With that said, he's locked up now on the cheap until he's 30, so they've got themselves a great deal if he gets better.