Is it worth going to NICMAR after 4 years of experience?

An experience of 4.5 years is excellent in its own way. Just be sure about your current and expected CTC during the placement process. 

Getting the same CTC even after the post-graduation would be a waste of time. It's a gamble if you're willing to make it. There are students who join just after two years of experience and get better pay, so should you after your 4 years of experience.

If you wish to join to learn, then go ahead, but if the salary is your only concern, then rethink your decision twice.