Income properties are not in the same market as residential properties. Price is based in the income generated so for prices to decline rents have to go down. To try to evaluate the market before investing look at the area and find out if a major industry is going to shut down and the workforce move away then rents will fall. Rents fall when there is a glut of available housing. If there is allot of foreclosues and empty houses that can affect the fourplex because renters will move to the houses causing the fourplex to lower rents to get tenants.