I personally consider the overall condition of the car as opposed to the number of owners or number of miles on the odometer.
Take my e39 528i for example, on the face of it, its 270k+ and has 4 previous owners showing on the log book.
In actual fact, it was my Dads company car from new, the company changed ownership a couple of times hence the car changing registered keepers, finally, my Dad retired and kept the car (another change of ownership) and then I bought it, adding owner/keeper #4.
When in fact, its stayed effecievely in the same family since new.
As I say, condition over pretty much anything else, but generally, the fewer owners the better and I'd imagine 4 owners over 15 years is not bad going!