My four month old baby has smelly black tarry stools. she has recently started formula fully. very fussy and crying. any ideas?

Yes: Black taqrry stools at that age may be a sign of gastrointestinal disease and bleeding.. See your doctor asap.

Bleeding: Black tarry stool indicates bleeding from the intestines. The most common cause of GI bleeding in babies is milk protein intolerance. Your 4 months old just started formula which contain cow milk protein that may be causing black tarry stool. Need to be evaluated by your doctor.