Splitting Rent

Scenario: 4 people lease a four.bedroom. each person gets own room and the rent is split 4 ways.

Person 1: decides to.sublet room for 2 months. Subletter responsible for paying person 1rent portion. Rent is still split 4 ways

Person 4: SO wants to move in. Everyone gets along well. SO will share room with person 4

Person 3: rent should be split 5 ways.should SO move in.

SO: disagrees because he/she is not getting personal room and this is.not the case for subletter. Thinks that person 4 rent should.be split

Who do you agree with? Please state reasons.

Edit: Utilities would be split 5 ways. The debate is over the rent.