A new BMW X4 is about to replace a four-year old predecessor. Are short life cycles the secret to quiet, steady success? That could be why Subaru of America has climbed ten places in as many years and is closing on Hyundai. BMW may be experimenting with this idea, adding to its recent progress: last month, Mercedes-Benz USA was pushed aside. And here in Britain, BMW was the UK's number two brand in June, outselling Ford.