I have an issue with my IPAC4 where player 3 and 4 do not work occasionally.  Unplugging the ipac from the pc and plugging it back in solves the issue.  I have the older version too.  Could be the same for you or a wiring issue.  Hasn't been frequent enough yet to send it back to Andy to look at.  Trying unplugging/plugging and see what happens.