"would use 10' 4"x4" post and set 3' into the ground leaving it 7' to top of sign." is about right.
We have done a lot of these too and for only one 4x8 ( horizontal I assume) panel you could get by with 2, 10' 4x4 posts and maybe a stringer or two. Unless you are WAY out in the boonies you have a sign code to deal with. Codes limit where you can install, who can install (need a license) how long you can keep it up, ROW setbacks, how high it can be, the size and (in some places around here) limits on how many colors and typefaces you can use.. You will need a permit.