Thanks for using DoctorSpring.com for posting your query.
I have gone through your question in detail and I can understand what you are going through.
Firstly I would like to congratulate you for your stable illness of bipolar mood disorder and the credit goes to your proper medicine compliance.
Regarding the fourth child, I would like to discuss the pros and cons. For having a pregnancy, you will have to discontinue Lithium (ideally). There may be a risk of relapse in this case. Moreover, if you continue the pregnancy with Lithium, there is a risk of congenital anomaly to the child (Ebsteins anomaly). The risk is low but if this happens then the burden is big.
What I personally feel is, you may go ahead with the fourth pregnancy, but I would suggest that you gradually go off lithium before pregnancy. As I said there is a risk of relapse but if it occurs then the medicines can be restarted and the risk can be accepted.
Hope this helps.
If you have any further query, I would like to help you further.
Dr. Srikanth Reddy MD