Does any one sell a 4 inch down pipe for the ats turbo? it is a new turbo that I have a 3 inch one on now to 3.5 inch straight pipe, but i dont like the sound. When i get on it it still raps like a big block. I want to go with a 5 inch exhaust. thanks.

 

Never seen one marketed.
Unless you have a body lift installed, I do not think you will get a four inch downpipe in there.

For what it is worth. If you have a large electrical contractor in your area, they may have a bender for 4" EMT conduit. That conduit is the same size as a 4" tailpipe. The conduit benders make nice bends that do not smash down on the inside radius like muffler shop benders do. The radius will be longer than muffler shops can bend, but the bend will be close to a mandrel bend as you can get. A down pipe would be easy to make.
Take your 3" downpipe and a stick of 4" tailpipe and a 50 dollar bill and you can probably have a 4" downpipe.

I think it would be easy to get a 3.5 to 4" increaser and clamp it on the outside of the turbo outlet flange, then attach the 4" downpipe to that. The rest is easy.