4x4x8 will fit about 3 tons usually mounded up a little. 1 ton of coal is approximately 40 cubic feet per ton, might be a little less, might be a little more.... The rice will take up more space than the pea.
Anything sold by weight in Pennsylvania, (coal, rock etc.) must be accompanied by a certified weightmasters certificate with the following things list: Date and time, truck license, scale location, tare, net, gross and weightmasters signature. If you're splitting loads you need a weightslip for each seperate load. They are also required to keep a copy on site at the scale for X amount time. I do not know what the laws are in N.Y.