4" or 5" exhaust ?

I would get the 4" then take it to a muffler shop and have them expand the end to your 4 1/4" with their flaring tool, then take it home and slide it on. You can get a clamp to hold it or weld it. What ever you prefer.

Hi, Tacacat.
Me being me, I'd go for the 5" option 'cos turbos LOVE being able to get rid of their used exhaust gases without restriction. Russelln's suggestion of going to an exhaust shop to make whatever pipe you choose fit your 'manifold' is a good one. How-wevver, I'm guessing that you are planning on fitting this pipe on the outlet pipe of your turbo rather than the manifold and welding it to either would NOT be a good idea. I'd suggest that you stick with the heavy duty clamp similar to the original Cat one. To do this properly, you will need to cut 3 or 4 clamping slits in your pipe to allow the clamp to tighten the pipe down onto the outlet tube.

just my 0.02.

I would get the 4" then take it to a muffler shop and have them expand the end to your 4 1/4" with their flaring tool, then take it home and slide it on. You can get a clamp to hold it or weld it. What ever you prefer.

If the stack has already been slotted it will crack at the slots when expanded .
I just had a stack made for our 120G grader .The young guy making it slotted the pipe first before using the expander ..His boss saw what he had done and made him cut a new piece ,explained to him , the reason .As a 4 inch stack will already be expanded and slotted ,there is a good chance it will crack or split when expanded .

I would go with the 4",its only got to be expanded 1/8" for it to fit,a clamp to secure it is the way to go.if my memory is correct the turbo elbow is a cast iron,welding an exhaust stack on does not make it easy if the hood has to come off.
GC 👍