Best Buy started offering the high-end Sony 4K Ultra High Definition TV sets in 700 of its U.S. stores on Sunday in an effort to shake up the stagnant TV category. The TVs — which have 8.3 million pixels compared with the 2.1 million pixels on regular high-definition TVs — are purported to have four times as sharp a picture as traditional HD TVs. They’ll be showcased in Best Buy and Magnolia stores’ home theater departments, and executives say a demo will make all the difference in convincing consumers this is the next big TV trend. The set will also do its best to “upscale” TV, DVD and Blu-ray movies, so they look better. “It’s a marked step up in quality in terms of picture improvement,” said Mike Mohan, president of home for Best Buy. Sony Corp. and other manufacturers, debuted pricey 4K TVs at the Consumer Electronics show in Las Vegas in January, but those had 84-inch screens and cost $25,000, out of range for most people.