Find 1 four-bedroom apartments for rent in Mercer Island, WA. A four-bedroom apartment is a unit with four distinct bedrooms that share a common area and kitchen. Depending on the floor plan, each room may share or have its own bathroom. If roommates split the cost of a four-bedroom rental, it can be a very affordable option. Additionally, units with more bedrooms typically offer a larger living space, allowing renters to make room for shared furniture or host guests.