The answer varies widely, but in average cars are usually driven 12-14k miles a year. Anymore than that is considered high mileage. A 4 year old, assuming normal use, shouldn't have more than 54k miles on it max.
Also, the more important thing is to check out the service history. A car can have only 10k miles but on the verge of breaking down, while another with 60k that's been driven carefully, religiously serviced and maintained can expect to live another hundred k miles with no major problem. The way the owner drives matters, a lot.
As for deductible, it also varies by make-model and your area. I don't know much but I've been told that the maximum you can deduct is 2% off vehicle price for popular models, and up to 3% for less popular ones. Unless the mileage is ridiculously high, think 25k a year and over, then you can deduct by about 4-4.5% off price depending on model. No concrete source here, just what my uncle (who used to work at a Honda dealership) told me.