<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def intersect_4(self, second, third, fourth):
 """ Intersection routine for four inputs. """

    self.intersection(second)
    self.intersection(third)
    self.intersection(fourth)
    self.coalesce()
    return len(self)