Keeping in mind what you have said about this being a mock-up, I would think a little about how you intend to incorporate your 4x4 legs.
It's amazing how much weight a workbench will be asked carry, especially when it's used for metalworking. Just as a suggestion, the 4x4's need to be under the bench framework, not attached to the side of a 2x4. you could possibly use 2 2x4 crossmembers side by side and set them on top of the 4x4 leg or, better yet, run 2x4's, or another 4x4, lengthwise under the top framework and set them on top of your 4x4 legs.
Have you decided on benchtop material? Over the years, I've had anything from plywood tops to steel. Currently, I have a steel topped bench; nice for keeping clean, oil spills and such aren't a problem, no cracks for small parts to hide in, and it's non-combustible. It's also very noisy, and unforgiving to parts accidentally dropped on it.
I did have a maple (butcher block) bench also, and really liked it for assembly work; but when I moved my P&W lathe in, it had to go.
A large shop I worked in made all of their benches in-house. They used 2x6's, laid flat, lengthwise on the bench and covered them with hardwood flooring run front to back. Very nice to work on, and if they were damaged, they could replace the flooring in short order.
Kevin