RE: New LiPo defective
Chop out the bad cell and use as a 3cell. You will need to replace the balcance lead with a 3cell one. It is for this reason that when my packs die (naturally or otherwise!) Ichop all the leads off and keep them.
You are probably better getting another pack- think hobbyking do warranty these for a month after they ship them, so you gotta be quick as you end up with a week to test them (also I think you have to add their serial number somewhere on their site. I'm in Scotland so they never arrive in time for me!)
Good luck