What Kind Of Nails For My Carport?

Or screws? I'm making some modifications to a car port basically to raise it. I'm going to be doing a notched pressure treated 4X4 with a pressure treated 2X6 as the perimeter of the car port. Hopefully that makes sense. What kind of nails should I be using to attach the 2X6's to the notched 4X4's?

TIA

Carriage bolts

Really? Any idea what size?

Galvanized 4 ½ inch coach bolts?

First tell us HOW the 4x4 and 2x6 relate to each other AND the existing car port framework.