as I am right in the middle of deer-fencing an acre, I have some comments:

4 feet is a deep hole to dig with a post-hole digger, can't really be done, you'll have to open up the top two feet with a shovel. IMO, it is un-necessary. 2.5 - 3 feet, the max depth with a digger, is plenty adequate if you tamp well.

I ran heavy gauge wire around the tops of the posts, stretched tight, to hang the fence from. Works well. You can measure up and staple the wire at a set hight from the ground.

Personally, I think you'll be disappointed if you use chicken wire fence. It is unsightly, never straight, and starts rusting pretty fast. I'd recommend welded wire utility fencing, which comes in all different heights and lengths. The holes are 2"x3". which keep everything out except baby rabbits, which for that just fastened 1/2" mesh hardware cloth along the bottom 2'. Squirrels can climb pretty much anything, so you can't really keep them out, but they aren't much of an issue. Woodchucks are horrible little things for any vegetable gardener, and are extremely difficult to deal with. As you said, they dig under stuff, so you'll indeed need to bury fencing outwards from your threshold. Chicken wire is ok for this because it doesn't need to look good. Just bury it a few inches deep, but straight outwards. They'll attempt to dig and hit the wire, and stop there.

As for the post depth, well, here in Wisconsin, if you DON'T go 4 feet down, you'll be sorry after a winter or two. You don't want to bury something above your frost line, or it will get heaved out. I don't know how cold it gets in zone 7, or how deep you get frost, if any. Also, if you have loose soil, your posts can be wiggled loose easily if you're not deep enough. Have fun, and take your time; no need to rush and then be unpleased with the results. I had a lot of fun building my fence, because it is like a work of art to me, but I'm really anal about building things perfectly.

Joe

Lots of post holes, I would rent an auger. If you want to dig them manually you can break it up over a few days. You don't want to dig holes all day.

Good luck...post pics of your project!

First of all, thanks to everybody for the feedback. I looked at prices for the hardware cloth, but wow, it's pretty expensive compared to chicken wire. The chicken wire from TSC is galvanized, so hopefully rusting wouldn't be a problem.

Regarding how far to sink the posts in the ground. Here in my part of Maryland the frost line is apparently about 18" -- building code says to have footers 30" down. I suppose I could go out in the yard and dig a trial hole to see how far I could get down. Or I could just order the 12' posts and get down as far as I could and then trim the post to size once I give up on digging any deeper than about 2'. I assume the cost difference between a 10' and 12' post is small enough to not worry too much.

Here's another question: for the section that's buried. I was thinking of burying it straight down. Should I angle it at a particular angle? Or should I go down a certain number of inches and then out at a 90-degree angle from the post? TSC has a 150' length of 4' chicken wire, so I only have a foot to bury unless I change that I would buy.

I think Joe is right about the chicken wire, the 150 ft roll is going to be REALLY hard to manage, it will deter, but if you have a determined critter they will go through it. It is made to keep chickens in. Raccoons destroyed my garden one year we didn't get more than a dozen ears of our corn, and they will rip right through chicken wire.
Also hole depth 2 1/2- 3 ft seems to work for central Va horse fencing
Good luck!

You do not have to bury those posts 4' deep in Maryland. By far the worst garden pest is deer -- and your fence can't be too high. 8' is barely enough. If you've got the post height, and it sound like you do, use as much as possible for deer deterrance. The rest of the critters don't do nearly the damage deer do.