One fire empties every unit
On a single rental house a total loss costs one rent. On a fourplex it costs four, simultaneously, for however long the rebuild takes. Loss of rent limits set from the rent of one unit are the most common and most expensive fault on a small multi-unit policy.
Shared systems spread a single failure
One roof, often one water heater arrangement, frequently one electrical service. A failure in a shared system is not contained to the unit it started in, which is why carriers underwrite the building rather than the tenancies.
More tenants means more liability
Common areas, shared stairs, shared parking and more people coming and going. The liability limit appropriate to a single rental house is rarely the right one for a building with four households and their visitors in it.