4 inch NPT Male to 3 inch NPT Female Adapter. Stainless Steel.
If you want to use 3" metal pipe connecting to a pump that has a 4" discharge, this is the adapter for it.
Note that the 3" pipe will create noticeable friction loss for over 250GPM running through a 300+ feet pipe. Use our TDH calculator to determine if you want to use a 3" drop pipe instead of 4".