As soon as I read your post, I thought "threaded rod", too, but the others beat me to it. Here is one thought not yet mentioned:
In many places, it is good practice to separate the wood from the concrete. Where I live, if we were to put the outer 4x4s posts 12 inches directly into concrete, then the mailbox stand would break away in a few years at most. We use metal supports set in concrete, and attach the post to the metal supports with a slight gap or a non-wood spacer.
You can find the metal supports at your local big box store. The ones I use have a wavy metal plate for the concrete, and lots of bolt holes for the wood. I like the Simpson Strong Tie brand, which include the appropriate bolts with the product.