My alternator has got to be changed . Car only done 11.249 miles

Thankyou.fiesta Ecco boost 1.000,has dark windows spoiler all extras what ever that’s called. What I want to know please the alternator has gone according to Allen ford, and spring on left back side of car, car has only done 11,239 miles, is 4yrar old December the 10th. So Reading a little about alternators today should last at least 7 yrs ? My car is not done the mileage or usage to justify this alternator going.Id this tight what I have read ?if do what can I do about it. Regaurds Tracey P