I recently emailed a ebay distributer on a 4inch MBRP exhaust, and the price for a turbo back. He replied, saying a 4 inch turbo back is impossible. Only Up-Pipe back is possible. Is this true?

Most exhausts that are called turbo back are actually down pipe back. The downpipe is not replaced, atleast it wasn't with my turbo back.

MBRP doesn't make a down tube, but several of the forum vendors do or can get one.

Will a downpipe back still offer a low-EGT environment like a turbo back.

Yes. Most people dont mess with the down-pipe and just go with a down-pipe back kit. With this setup you can expect around 200 deg. drop in egt's.

A downpipe back will do what you need.

Yes it is posable to do a 4" down pipe :D just takes a few bucks to buy one from TTS or some time to make one your self.