If a woman has been diagnosed and treated for schizophrenia in the past but has been well for 4 years, would it be wise to have a baby?

Hard to say. You may be asking either or both of two questions: is the child at risk for having schizophrenia, and what might the effect be on the mother. With respect to the former, the risk is about 6 times higher than the general population (6%), so not horrible odds. With respect to latter, having a baby is quite stressful, so be sure support systems are in place. Talk over in advance with doctor.