My 4 month old son who is breast feed no solids had black strings in his poo?

Usually no concern: Ah, baby poo. It consumes much of parent's time, and often creates anxiety, especially in the early years. Fortunately, there about as many normal variations of stool appearance as there are flavors of ice cream (ok,bad analagy ) . "Black strings" are no exception, assuming all other things normal. Breast fed infant stool is generally dijon mustard consistency but can vary. Mention it to his doc

Needs evaluation: Pl take your son to your pediatrician for an evaluation to rule out medical condition that needs toBe treated. If your son is growing well. seems active and otherwise OK, element of milk protein or lactose allergy needs to be ruled, that can cause something similar. Good luck:)