I would use sheetmetal, not alum. Alum and exhaust do not go well together. The fumes that come off of alum under extreme heat are toxic. When we machine alum in my machine at the shop...we have special filters we use in order to do it. Just letting you know.
Trace out the flange on your test pipe, and then your 4 inch flange and post them up. I think a thin sheetmetal gasket will help you out.