Can I get 250 above on the JEE Mains by preparing for 4 months ?

Unless you are a certified genius, no. By a certified genius, I mean you should one of those 'Beautiful Mind' type students who usually have an eidetic memory. Four months is a very short time and I simply do not see you cracking an exam like JEE Mains in 4 months. If you're really motivated, try next year. If next year is not an option, my advise would be to aim for a score between 160-200. You should always aim for the best, but don't fool yourself into thinking you'll score 250. A score between 170-200 will get you a decent NIT if not a great one. I wish you the very best of luck for the challenges to come.