Hello Adam!
I bought a 10' length of 4" exhaust tubing and a 45 degree elbow from a truck supply store in Memphis about a year ago. The pipe was $3. 90 per foot and the elbow was about $20. 00.
You should be able to find this at most any truck equipment store. NAPA also had it but the price was higher.
I recently hit on a great deal on 6 Navistar exhaust and tailpipes that have an amazing number of various bends in them. These were new surplus from a dealer and they came up to a little over a thousand dollars list price. Some were for DT applications, some for Detroit and some for Cummins M series. One of them is about 6 feet of straight tubing with a 45 degree bend on one end. I got the whole batch for about $35. 00 including UPS shipping. All the bends are smooth mandrel bends and the pipe is all excellent quality. These pieces are the basis for the 4" system I am starting to build now. One of the pipes even has a slight bend next to a 90 degree bend that will work as is for the downpipe. It already has a 1/4" pipe thread bung in it for use with a thermocouple for a post turbo pyrometer. The Ebay seller had them listed by application and showed a picture of each one. He had about 15 or so pipes listed and I simply asked him which ones were 4" and bought all of those.
Like I mentioned, check around for truck equipment dealers and you will find what you need. Call several because I found quite a variation in prices. I can not return the 10' section of pipe I bought due to the store being under new ownership. If you have a problem getting the pipe locally, send me am Email and it might be feasible for you to buy this section.
Good Luck!
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