Create a 1x4 brick named "chance". Place four 1x1 bricks on top of it, and another one as a switch somewhere else.
Switch:
onActivate -> <Named Brick: chance> -> spawnProjectile ["0 0 10", "Radio Wave", "0 0 0", 1.0]
1x1 bricks
onProjectileHit -> ....Events with 1 in 4 chance here....
Use different sized bricks for different ratios.