I have found several articles etc. talking about ensuring the feel seat fits the rod.

1st - does anyone know of an article that mentions reel seat sizes in building freshwater fly rods?

2nd - I am building a 71/2-ft; 4wt rod on a RodGeeks SERIES - C4F764.4 fast action blank to be sold at a "Black Tie" fundraiser @#$%& for a group that raises money for Smoky Mountains National Park upgrades and repairs the National Park Service cannot fund. My thoughts are to use a down-locking reel seat. I am guessing that the rod will be used primarily in the small streams & rivers flowing from the smoky Mountains in TN, VA & NC. I may use an aluminum/graphite seat or all aluminum. The handle will be a 7" Half Wells made from foam core and will be marbleized to match the rod colors. I will use Minima single foot guides for stripping and running.

Please tell me your choice of the reel seat size for this rod.

If you take a look at some of the suppliers on the left, specifically, Batson, Mudhole & Pac Bay, they list reel seat appropriate for 4wt rods. You will find a myriad of choices.

As Phil said, they give suggestions for the rod weight in the catalogs, after that you only have to pick an insert with a big enough ID the fit over the butt diameter of your blank. All the graphite insert reel seats I've used over the years had IDs well above what you need.