Conventional wisdom is double diameter for passive intakes.

So, you might want 2 x 4" intakes to accommodate a 4" exhaust. Bear in mind that bends in the ducts, reducers, and "flex" style ducting add significant "drag" on the system. I'm not sure if your 4" exhaust reduced to 3" counts as 3" or as 4". What size is the fan, and is it designed to "push" or "pull"? (The way your box is set up, you're asking it to "pull.")

Except: 80cfm is such a low flow that your drag may be negligible. However, if you ever need to add odor control, a bigger fan's gonna be needed to drive a carbon filter.

Those intakes/ exhausts are light traps also, yes?

You are going to hate 4' fluoro tubes as side lighting if heat is a problem. For one thing, the diagram shows them also lighting the top of the cabinet, not just plant material. Fluoros do throw some heat, it's just spread out over a long tube. But close 'em in a cab...

I'm curious about the color combo, too. Are you vegging or flowering?