Are you planning to use pin-together corral panels? If so, you only need a post on each end. For the mid-run, just pin each panel to its neighbor.
I wouldn't use 4x4 lumber for a post. I mostly use 2-7/8" oilfield tubing (usually in an H-brace configuration) for hanging gates and holding tension on barbed wire. For a standalone (unbraced) post, I would want 3-1/2" or larger steel.
If you do go with a double gate, I would recommend letting the gates overlap each other where they meet. A foot or two of overlap in the middle will add strength and stand up better if/when an animal tests the gate.
There are many ways to skin the fencing/corral cat. More details on what you're trying to accomplish would likely yield better feedback. It's not clear (to me) how you'll be using this piece of fence (i.e., what animals, how much pressure).