Montana WG, I agree that a 4x6 raised bed will not be a good option. The reason is that rhubarb is a heavy feeder of nutrients, a very heavy water user, and needs more than a 2x2 square because of growth habit. Also be aware that as a perennial, this plant will take 2 years from divisions to produce, and you should always leave at least 2-3 stalks after harvest to maintain plant health.
If you have some space on plain ground, this is probably a better option thAn a raised bed. It likes well drained soil, and doesn't need to be babied. Don't over-prepare or amend the site-- just give the division a foot square and 6" deep area of turned soil. You can add some organic matter if your soil is very sandy, but this is not a fussy plant.
Laurel