Should I write CAT with 4 years of experience (as of now).My main Target is NITIE, IIT Bombay or FMS?

Your choice to go ahead with this is a good one. There are quite a few reasons that will help you become successful if you plan to give the CAT after gaining some experience:

• A lot of students from top B-schools gain admission after doing some work and gaining experience. You can even try for XLRI General Management which requires 5 years of work experience.
• If you are planning to go for a PGP-Ex, let me tell you that this will not help you make any domain changes. But, if you still bag an admission, you can do a degree in operations and consulting.
• The institutes mentioned above are quite biased towards engineers. They will not pay much attention to your profile. So, if you are one, your chances of getting in are quite good. You can also give the GMAT for your future prep.
• If you are already working, with a degree from a reputed B-school, you will surely get an increase in your salary. You will also get the validation you need in your future endeavors.

In conclusion, this method of pursuing your further studies might be a little difficult in the beginning but you will realize that the pros are much better than the cons. You should think of going forward with this and take a chance.