Wait a minute, a 4 x 4 grid? That doesn't seen too likely somehow ... maybe you mean a 5 x 5 grid??
Eh, it works if you correlate the blast position to whichever quadrant of the board the captured piece happens to be in. The b0rkenness is probably just related to the thought of one capture killing a dozen pieces or so.
As I recall, the capturer got to pick the direction in which the thing expanded. Of course, this contributed greatly to the bah-rokenness.--MicahSmukler