Mileage IMHO is worse unless it's outweighed for a buyer by years because they just have to have some feature that's only available in a newer model year car, e.g. newer iDrive, Combox, etc. Lower mileage doesn't necessarily mean that it was only ever driven hard. 25K miles in 4 years could be someone who worked from home and/or had the M3 as a weekend car.
Most issues crop up on a car with miles, not time. Manufacturers come out ahead on warranties for precisely that reason -- most customers fall out of their warranty periods on time, not mileage.
Between your options, assuming both cars had the same options and same maintenance and driving patterns (if I could somehow know that for certain), I'd pick the 4-year-old car. But if you're considering changing how you use your car to optimize for resale value, you're missing the point. Go out and optimize for your own enjoyment!