4 foot post holes are ideal, 3 feet is fine, the frost line rarely goes deeper, especially the last few years. Use 4x6 posts instead of 4x4's if you can, they hold up much better and don't warp as easily. You don't have to use concrete, but if you do decide to use it, dry pack the holes, they sell concrete specifically for post holes. Dry pack just means you pack the mixture in dry, then wet it, this makes is 10 times easier to do the job (read the directions).
A down side to concreting the holes is the posts will rot out quicker, because water will seep into the space between the post and concrete and get trapped, shortening the life of the posts. Always put a couple of inches of clean gravel at the bottom of the hole. If you chose not to use concrete, you will need to pack the soil down very well, do it in stages. Keep in mind that a post just packed in with soil will not be as stable, but generally they hold up fine.
If you use pressure treated lumber boards, keep in mind that it will shrink up to 20%. If you paint or stain pressure treated, it needs to be reasonable dry, which can take anywhere from a few weeks to months from the date it was manufactured. You must sand the boards before staining or painting, this is because there is a coating that occurs during the treatment process which will interfere with adhesion of the finish.
Get yourself a post level (it has a bubble on two sides) it makes the job much easier. Only use galvanized fasteners if you plan on being around for a decade or more, deck screws rust even though they are painted. Construction screws are not suitable for outdoor use, but they are very commonly used anyway.
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