I agree with Frillylily: I would never use artificial mulch. Even stones (pebbles), paving stones or heavy-gauge black plastic film would smother weeds, but they starve the soil.
Besides leaching possibly toxic chemicals into the soil as it breaks down, artificial mulch adds no digestible organic matter to the soil - that is, it starves the soil and favors weeds like horsetail over desired plants.
What you use for mulch depends on what is cheap locally. There are many evergreen trees in WA, and some lumbering, so pine" or evergreen bark is cheap here.
Bark lasts longer than wood of a similar size, so it feeds the soil slower than wood of the same chip size would.
Big wood chips are probably second-best after bark. As long as they only sit on the surface and are not turned under, they don't cause nitrogen deficit as they break down.
I think that even TWO inches of the above would inhibit weeds somewhat. Would FOUR inches inhibit twice as many weeds? Or reduce the number of surviving weeds by a factor of two? Maybe and maybe not, but four inches will make pulling weeds easier than two inches!
Just don't pile up four inches of finely-shredded or water-retentive mulch. It might absorb all your rain and keep it from reaching the soil. And it might even pack down so tight that it slows down air's access to the soil. Well, probably not, but there is such a thing as too-deep mulch, if it is also too-fine.
If you have lots of leaves, pine needles, or straw, use what you have! But these will have to be renewed every year or even twice per year to keep weeds down. The upside is that they will feed the soil rapidly and soon it could be a beautiful flower border (with sufficient work).
I guess the following is not an option if you want it to be pretty AS you set it up. You can buy bags of mulch and lay the bags down ON the bed, touching each other, for a week or two. That will deprive the weeds of sunlight and kill some before the bark goes down. When you have all the bags in place, you can go along slitting them along a long side and dumping the mulch into place, uniformly spread by the bags' spacing.
Of course, buying mulch in bulk is cheaper than buying bags, especially if someone will deliver for free, for a church. Or a member might loan a truck.
12'x30' = 360 sq. ft.
2x4'x6' = 48 sq. ft.
Total: 408 square feet.
ONE inch deep = 408/12 cubic feet = 34 cubic feet = 1.26 cubic yards
(one cubic yard = 27 cubic feet)
TWO inches = 68 cubic feet = 2.5 cubic yards
FOUR inches = 136 cubic feet = 5 cubic yards
