As I dont understand every aspect of the Residential tenancies Act (about 140 pages on the web!) I am happy to stand corrected on this , but..
.. if you register this fifth person ,are you not now declaring him as your tenant with all the tenancy rights as per the Act ?
If you're OK with the fifth person ,although you're a bit annoyed about the cheek of it, then accept it. As a previous poster says ,be happy you're getting then full rent.
But, of course, the other three tenants should be happy as well. As they're paying less they probably are, but have you checked this.
If you are unhappy about this fifth person then write ,stating that you cannot accept any more than four persons ,other than temporary guests of registered tenants.
And do not register him.
Keep everything in writing so it is clear that you were very reasonable in asking him to vacate the premises..
But really all this is a bit late - even the shortest written agreement would have helped.