I tried using a 4x4 post for a 6-foot gate and it sagged in a month
I figured the extra thickness would be stronger, but the weight of the cedar planks was just too much for the hinge side. Now I'm looking at a full rebuild with a proper 6x6 and a diagonal brace. What's your go-to post size for a heavy single gate?
Ever think about using a steel post inside the wood? @johnrivera is right about the 6x6, but I sunk a galvanized pipe in concrete and boxed it with cedar. That brace is key, make sure it runs from the bottom hinge out to the top corner of the gate.
OH MAN, I just read this article from This Old House that said basically the same thing! They tested a 4x4 vs a 6x6 and the 4x4 twisted like a pretzel under a heavy gate. I tried using a 4x4 for my 5 foot gate once and it sagged so bad I could barely close it after six weeks. That diagonal brace is everything, I learned the hard way too. Now I always go with a 6x6 set in a good 24 inches of concrete, and I brace it from the bottom hinge to the top far corner like you mentioned.
Well, there's your problem. You tried to use a telephone pole for the gate itself. That 4x4 post was just a fancy lever, and gravity always wins that fight. For a six foot gate, I wouldn't even think about anything less than a 6x6 post set deep in concrete. The diagonal brace is non-negotiable, too. It's the only thing that stops that sad, slow swing into the ground.