Landlord's permission is advisable but not compulsory. The criteria here are overcrowding and sub-letting. As there will be up to 5 of you in a 4-bedroom house, overcrowding won't arise, and since you won't be sub-letting but presumably adding your fiancé as joint tenant, the second situation won't exist. Other than a letter from your sister, just enclose details of the property size and how many occupants. Ideally, try to get a letter from letting agent that they have no objection to your financé being added as joint tenant subject to credit checks. Tell them you need it for the immigration service.