While a neuter or spay can be done at any time the recommendations as to when is the optimum time vary. I think that the best time is to plan the surgery just prior to puberty. For a puppy like yours I feel that would be between 5 and 6 months of age. Early neuters do not allow the positive influences of hormone production to occur and seem to me to add to the retention of baby fat and problems with weight control. Maturation of the musculoskeletal system as well as full development of the urinary tract do not occur they are at least exposed to the early production of hormones generated by the ovaries or testes.