It all depends on how you intend to collect the rent:
If you rent the house as a single unit for x euros per month, the tenants usually organise one person to pay that sum into your bank account. If one tenant vacates, those remaining still pay the full rent - thus you always get your x amount per month. In this case, all tenants are jointly and severally liable for the rent.
If you decide (and may get a little more per month) on having each tenant with his designated bedroom and signing his own lease, this will give you more work. However, if one tenant vacates, you may lose his rent until you have filled his place.
IMHO, the former is the easier and better option. A group of tenants who know each other is usually easier to deal with. If one of the vacates, the group will try to find a replacement who fits into their lifestyle (though subject to your vetting criteria and reference checks) where as if the rooms are let on an individual basis it is your responsibility to find the new tenant who may not fit in with the lifestyle of the others.
The choice is yours. In all cases, the lease agreement would be virtually the same with only minor differences. However, if you go the individual route, you would have 4 separate tenancy agreements and therefore 4 registration fees to pay to the PRTB whereas if renting the house as a single unit, there is only one tenancy agreement and therefore only one PRTB registration fee to be paid.