I have 4 years of experience in SCM. If I do an SAP MM certification, how will it be helpful for my career growth and opportunities?

If not, that is what you should aim for....APO is a little narrow compared to MM but has much higher paying opportunities. It is also much sophisticated in terms of algorithms, models etc.

SCM - A little screwed up term. Are you working in order management/fulfillment/Logistics or did you work in planning and optimization? If it is later, MM certification might not be of much help. If you are in planning and optimization, then are you APO (Advanced Planning and Optimization now called SCM) certified already? If not, that is what you should aim for....APO is a little narrow compared to MM but has much higher paying opportunities. It is also much sophisticated in terms of algorithms, models etc