Gap is for value, if you owe more than the car is worth. By now you should have some equity in your vehicle becasuse its been 4 years so you wouldn't need gap.
As to extended warranty policies for maintenace, that is a separate issue. I am not a fan because the policies are very expensive and generally have exceptions. IME its best to put the funds aside to pay for repairs that come up. Others probably will have different opinions.