Let's see. 4 million pounds is around 1800 metric tons. Density of concrete is around 2 tons/m³, so 1800 tons is the mass of about 900 m³ of concrete. This is a volume of a disc about 1 m thick and 35 m in diameter. Seems plausible.

Also, from Wikipedia:

Explosive steam pressure [...] destroyed the reactor casing, tearing off and blasting the 2000-ton upper plate, to which the entire reactor assembly is fastened, through the roof of the reactor building.

https://en.wikipedia.org/wiki/Chernobyl_disaster#Experiment_...