Are there any medicolegal cases, when medical staff has shown negligence in diagnosing 4 month infant testicular torsion resulting in loss of testicle. initial newborn screening showed both testis normal shape and size. and now after 4 month they diagnose

Torsion: can happen at any time so impossible to answer your question. I suggest you address this question to your pediatrician and ask for an independent review of your baby's clinical record before deciding there was a negligent component. To assist with that review, include any observations you may have had prior to the diagnosis. I expect there will be a reassuring explanation of the events.