Boy, 4, killed by stray bullet in Bx. gunfight, leading pol to back stop-frisk

William MillerPolice are investigating the death of a 4-year-old boy in The Bronx, who was killed Sunday after being struck by a stray bullet. A 4-year-old boy was killed by a stray bullet in a Bronx playground last night as two thugs, angry over a dispute following a basketball game…