Another leg Option
Our brackets were designed for standard 4" x 4"s which actually measure 3 1/2" x 3 1/2". Sometimes these are hard to find untreated. Another option is to use full 4" x 4" for legs.
These are available untreated from saw mills and are commonly used for fence posts. Pictured on the right is a bench with full 4" x 4" legs.
It looks more massive and is stronger. It also weighs more. By counter-boring the mounting holes on the nut side, the same 4" long 3/8" carriage bolts can be used.
This bench has plastic caps covering the holes for a more finished appearance.