I had three (gestational diabetes and therefore big babies).
My doctor did advise I not having anymore after DC3 as I was getting sicker with each pregnancy so I was sterilised.
I think it depends how well you are otherwise and how long you have had in between each C-section (I only had 13 months between my first two, doc not happy).
Are you sure you would need another C-section? I know it is likely but not always an absolute necessity.
If you doctor says it is safe I would listen to him as he has your case history in front of him.