I have a broken 4x4" fence post at bottom. Post is in concrete Wondering the best fix without digging up concrete and doing it all over. Thanks!
Here are some options for repairing your fence:
Get a $5 metal T-post or two and drive it in the ground near the broken 4x4. You may have to pound the T-post in at an angle to avoid hitting the concrete. Screw the 4x4 to the metal T-post. Easiest, cheapest, and fastest. This is a good temporary solution to a leaning fence.
Temporary brace the fence with 2x4's and detach the broken post from the fence. Using a drill and chisel, remove as much of the broken post from the concrete as possible. Get a new 4x4 post. Carefully measure the hole in the concrete. Use a handplane on the bottom of the new 4x4 post. Shave off about 1/2" from each of the sides so that it will fit in the hole you measured. There is no second chance, so make sure the post is smaller than the hole. Pound the new post into the hole in the cement. Use shims to level the post. Finally, reattach the fence.
Unless the 4x4 broke for a specific reason, like it was hit by a car, it's likely that rot caused the post to fail. If that's the case, it's likely that the other posts are rotten too. Check by stabbing with a screw driver. If the screw driver goes into the wood more then 1/2", it's rotten. Repairing a rotten fence (option #2) is almost certainly not worth it, as another post will likely fail in the near future. If the fence is rotten, it's best to prop it up (option #1) and then take care of it when the weather is more pleasant.
It really depends on how much "good" post you have to work with above the concrete.
If you have enough to attach another post to what remains of the original post, you could cut off the original post and join a new one to the "stump" using a lap joint or scarf joint, then run a couple bolts through it to hold together.
You should probably orient the joint so that it provides maximum resistance to wind forces exerted against the fence. Posting a picture would be very helpful in this situation.