MBRP 4in turbo back exhaust

[2004Powerstroke]

Hey guys, I was piecing together my new 4in turbo back exhaust and realized the down pipe was smaller than a 4. so my question is should the down pipe on a 4in turbo back exhaust be smaller?

 

[ghohouston]

Some kits have a 3.5", some have an actual 4" downpipe.