Chris,

Answer: Put it on a scale. :)

Seriously, we cannot answer your question without knowing what the cylinder is made from. The density of the material multiplied by the volume will give its weight.

The volume we can calculate:

Assuming the "thickness" is inwards (that is, the outside diameter is 4ft, but the inside diameter is 3 ft), we can subtract the volume of the empty inside from a full cylinder outside. The volume of a cylinder is the square of its radius times pi times the height. So the volume of your empty cylinder is (2²)(pi)(4) - (1.5²)(pi)(4). This is about 22 cubic feet.

If your cylinder was made of concrete, which is typically about 144 lbs per cubic foot, then it would weigh 22 x 144 = 3168 lbs. If it was made of styrofoam, then it would weigh 22 x 6.25 = 138 lbs, since the density of styrofoam is about 6.25 lbs per cubic foot.

Hope this helps,
Stephen La Rocque.