I would think that most new car owners will have their vehicles regularly serviced, to maintain the manufacturers warranty if for no other reason and this should catch any potential safety issues within the first four years.
If the car does average milage and is serviced at regular intervals by a reputable garage, I see no problem going over to 4 years.
I usually clock up about 75,000 in 3 years and tend to change around that time. The car is always serviced at it's correct milage but I feel that after 80,000 you loose more in garage fees than you do trading in for a new one. The only reservation I would have is those high milers who do not get the cars regularly checked.