For getting under 5000 rank in wbjee you have to score atleast 90 marks in wbjee for safe side .actually marks vs rank depends on type of question paper ,difficulty of question paper which changes every year .one year difficulty of questions may high and other year low so according to that marks vs rank is there .two scenario are in
2015 -85 marks is 4200 rank
2016–98 marks corresponds to 4000 rank