Should all coolant hoses be replaced every four years?

You are correct regarding the rubber hoses failing internally. They will degrade over time and harden. Modern quality coolants are designed to not affect rubber hoses; however old coolant will become caustic and degrade the internal rubber surface of the hose. If you maintain a regular service interval of your coolant including reverse flushing the system every 24 months you will lengthen the serviceable life of your hoses. It would be fair to assume that a well-cared for cooling system should be reliable for at least five to eight years.