Currently, I am working with a reputed company as a Manager with almost a total work experience of 3.9 - 4 years. What should I do?

You can do PGDMA course from a well-known universities after applying for CAT/MAT exams. If you are interested in teaching you can also do Ph.D. in your interested domain.

You could pursue MBA/PGDM part time or regular, if you want to have a Master's degree, which will surely help you in your career. Or, you could do a part time Diploma course related to your field, so that you can also work and side by side your knowledge in your field will be increased.