Scott, would that be Gwinn, Michigan? We aren't being busybodies, just need to know what your climate is like to give you good advice.
If you have a local plant nursery, they are the first place to go to buy your garden soil. Most nurseries sell "topsoil" which really can be almost anything, and they also sell "compost" which surely will be a lot more organic material than the topsoil. Go for as much compost as you can, and mix it with your existing soil if it's any good at all. It will be a little more money, but a LOT better value.
Some counties have a composting program at the landfill - ours makes fabulous compost that is free for the taking (but you have to load and transport it yourself). In Salt Lake City where my daughter lives, we got great compost from the landfill and they had a front-end loader to load it in your truck for you. But they charged $25 per load. (that's really cheap for good compost).
May I suggest you make your bed a little narrower than 4ft. ? It's hard to reach the middle of a 4ft wide bed, to weed, plant and dig. Once you prepare your soil, you really REALLY don't want to be tramping around in it, compacting the soil. Also, when you fill it up, pile the soil well up over the height of the sides, as you will get a lot of settling, and by the end of summer what was 12in. of soil depth will have settled down to 6in. or less.
Here are our raised beds at the local school - first picture when the beds were built in August, soil is just below the rims, second picture taken in October, the soil has already subsided nearly 3in. (this is also how I know that 4ft. is a little bit too wide . . . we built 4 new beds that are 3ft. wide and are SO much easier to work in).
