If the sign is semi-temporary, (real estate, etc.) 4"x 4" posts are sufficient. If this is a permanent sign, I would recommend 6"x 6" posts. I can buy only two kinds of pressure treated lumber in my area: straight and still wet, or dry and warped, twisted or bowed. I'm assuming this 4'x 6' will be at least 8' tall. There is a strong possibility your 4"x 4"'s will twist, warp or bow after install.
If it's a temp, we don't use concrete. Instead we bury the posts roughly or more than 35% of total height. 96" high x 35% = 33.6" minimum depth. There is a lot of clay soil in my area and the breaker bar does more work digging the holes than the post hole diggers. As we're busting out dirt with the bar, we slam it against the side of the hole, thus compacting the dirt around the hole. I got a giant book on the shelf about concrete, compaction and so forth, but I won't get into the math. We bring a 5 gallon bucket of gravel to add in with the dirt when we back fill. Dig your holes as close to level as you can get by measuring hole depth, while accounting for grade. If you have a grade, lay a 2"x 4" across top of holes. Raise 2"x 4" till it is level and measure the difference between the heights. Adjust hole depth accordingly. We use a breaker bar with a bell on the other end of the wedge. Pound the bottom of a hole with the bell so you have a solid bottom. Set your prefabbed 4'x 6' sign structure into the holes. It will probably be a little off level. Scoop a small amount of gravel into the hole that is too deep. Lift that side of the sign up enough to allow gravels to work there way under the post. Bounce that end until the sign is level. Once you have your sign level, roughly plumb it and scoop in a couple shovels of dirt. Use the bell end of your bar to tamp the dirt, thus compacting it. Repeat this process, adding 3"-4" of dirt and mixing in a scoop of gravel with each tamping. You should be compacting that 3"x 4" to about 2". Check plumb with each tamp. If the sign is leaning, tamp harder on the side it's leaning to. This will move the post back to plumb. Once you've reached the top of the holes, mound the dirt up around the post to promote water runoff instead water in you hole. We've installed many signs this way over the last 30 plus years and it works. It's hard work, but it's not rocket science.
Concrete is best applied as a ballast in pole/post installation. It's the weight below fulcrum (ground level) that anchors the sign. 160 lbs of concrete won't be noticed by the wind load absorbed by 24' feet panel. Again, I won't get into the math. The Great Wall of China is still standing because of compacted dirt.
If the sign is permanent, I highly advise 6" x 6" posts because of their stability. Same technique as above, but with sackrete or quickrete.
