I think it works out to 100 cubic feet.

Then you have the question of what kind of soil and amendments and that's a more complicated question.

Also, what will be under the beds? Are they open to the ground below? If you double dig the soil under the beds you will end up with plants with very robust root systems.

Its 100 cubic feet, so you'll need 33 cubic feet of vermiculite, 33 cubic feet of compost, and 33 cubic feet of peat moss. That might be a little costly-at least if you want to do it right with Mel's mix

Several (many?) years back, I did a same bed experiment with a newspaper bottom over undisturbed soil on one end and double dug on the other end and about 6" of mostly compost over the whole bed. I saw no difference in production from one side to the other after the newspaper had decomposed enough for roots to punch through mid-season and fewer weeds on the end that was *not* double-dug. I have not double-dug a bed again since then and have no qualms advising everyone else to skip that time and labor intensive task in a raised bed setting, especially if they are preparing a bed off-season that will not be planted in for a few months..

My compromise to the expense when initially filling a bed for immediate use is to use mostly bulk mushroom compost augmented from my household pile for some variety. I line the bottom of the bed with newspaper or cardboard, then a thick layer of old leaves and/or leaf mold (which has properties similar to peat moss when it breaks down), topped with the (hot) mushroom compost and then right around initial plantings I use a purchased potting mix (I prefer Scott's moisture control) to shield them some from the active composting going on around them.

The only vermiculite that gets in comes from old pot-plants and from rooting cuttings as the used medium gets dumped into the compost pile. The worms eventually mix up sand from beneath the beds for more consistent percolation properties. Over a heavy clay soil I might be more partial to using vermiculite, but over sand I don't need it and couldn't see much benefit over rock either.

My beds are four foot wide and 25 feet long, and the depth is 12 inches.

Math is your friend. =)

A cubic foot is a cube of dirt 12 inches long by 12 inches wide by 12 inches tall. Your bed is 4 feet wide by 25 feet long and 1 foot deep. Thats 4 x 25 x 1 = 100 cubic feet.