Apple turned in its best quarter, on the iPhone front, for four years, and grabbed top position in global smartphone shipments, at least temporarily, in the last quarter of 2019. The company’s unexpected revival, after successive quarters when the iPhone failed to deliver its normal levels of performance, will be very welcome to shareholders. Despite the recent growth in other Apple businesses, notably content and services, and its investment in enhancing revenue streams such as TV, it is still significantly over-reliant on a single product family. The iPhone uptick can be credited to several factors. Launching a particularly desirable new model for the holiday season is not one of them. A year earlier, Apple revealed the iPhone X, a major…