Going forward, the 4 tenants could require equal contributions (unless there is some written agreement setting out each person's contributions, which agreement is enforceable until it expires): after telling the 5th tenant that he/she has to pay an equal share, they could sue if she or he does not.But as for prior (to date) smaller payments, the 4 would only have grounds to sue IF there had been an agreement that the 5th tenant would pay more, which agreement he or she violated. In that instance, they could sue for breach of contract. But if there was no actual agreement with that tenant as to how much he or she would pay, then paying less was not legally wrong (it may have been morally or ethically wrong, but that is a different issue): the law does not intrinsically require fair or equal payments, so there is nothing legally wrong with paying less unless it was violation of an agreement.