There are different 4' disks, different in build, weight and over-all quality. The prices new range from $600-$900.
Since the Quad guys also want them for deer plots, the prices are screaming high, for used, and typically sell FAST!!! YMMV.
I just got to this one, a 12 x16" for $400. Offered $350 and the seller wouldn't budge, as there were a half dozen others lining up to look at it. Didn't blame the seller one bit. It is incredible. Had been looking for a few years.