Depends on your driving style.  for me, I'd leave it.  4 mm is a fair amount
of pad depth still.  However, you need to keep in mind that, again, depending
on your driving style they may need to be redone within a year.  Nevertheless,
for peace of mind, it doesn't seem unreasonable to change at this point. If
it were my wife's boxster ( I know here driving style :-(  ) and I wasn't as
aware of the car because I wasn't driving it much, I'd change it.

cheers. guenter  

Brake pads are relatively cheap insurance - if you could have gone a couple
of mm longer, over the life of the car you'll save what, a set or two of
pads? I'd change 'em, personally.

On the other hand, if you leave 'em and they wear to the point that you go
metal-to-metal, then your costs will be much more.