Hi Ajit,
I am very glad to know that you have 4 years of experience in production Planning and Control Engineer, normally its difficult to get people who has domain experience in these subjects.
As you have experienced in these areas learning SAP would be very good for you especially if you proceed to learn SAP Production Planning as well as Controlling module as Production Planning has very close integration with Controlling module for the areas of Product Costing as well as Profitability analysis other than Cost Center, profit center, cost element and internal order accounting.
So even you proceed to learn Production Planning module make an attempt to learn controlling module as well as much as you can, both are very nice combination and its very rare to find this combination and industry eagerly needs these kind of people who can be ideal resource for manufacturing industries..
To answer your question whether certification is necessary, please remember certification is not the passport to enter into SAP or even to get a job as SAP Consultant as industry needs people who are knowledgable about process and talk and understand process as SAP is just a tool or software that helps to achieve that process running or execution.
So off course you can do certification but you can also take sap training from any institute of your choice and you will be assumed in the industry as trained hier or lateral hier from sap perspective.
At last I am happy to say that we need people like you who are knowledgable about business processes as because of domain knowledge but only need to know how to achieve those business processes using sap instaed of just becoming a by hearted sap configuration or functional consultant.
Wish you good luch.
Let me know in case you need any clarification and I would be happy to help you.
You can join me in linkedin if you like.
Thanks & Regards
Subhasish