That... Is a bit different, as the shockwave doesn't seem to be omnidirection (ie it only seems to destroy stuff in a line, not the up and down too). That'll probably require a different calculation if that's how it is.
However, assuming it is a normal shockwave (which I'm doubtful about), and considering it is in air, it'll be;
((x/0.28)^3)/1000 -> ((4/0.28)^3)/1000 = 2.9 megatons, then divide by two, which will be 1.45 megatons of tnt, ie Low 7-B.