yards of concrete for a 32 foot circle 4" thick

You can do this in 1 or 2 steps:

You can use the volume of a cylinder formula:
V=Ah
where V is volume, A is area, and h is height*

--or--

First find the radius using area of a circle formula:
A=(pi)(r^2)
A is area and, r is radius*

Then use the more complete volume of a cylinder formula:
V=(pi)(r^2)(h)
where V is volume, r is radius, and h is height*

*make sure to convert so that all your numbers are in the same unit before plugging them in (4inches=0.333ft=0.111yds)- if you want an answer in cubic yards plug in r and h in yards.

Hope this helps!