RadioShack said Tuesday that it would close up to 1,100 poorly performing stores amid widening losses, or roughly 20% of its locations, as part of an effort to remake itself for a more competitive era of electronic retailing. The store closings will leave about 4,000 locations in the U.S. The company has been trying to revamp its stores for the past year. Yet some analysts say the retailer faces an uphill climb as it struggles to compete against larger big-box stores such as Best Buy. RadioShack stock (RSH) was down about 15% to $2.31 in afternoon trading. In the fourth quarter, the company lost $191.4 million, or $1.90 a share. Revenue fell 20% from the same period a year ago to $935 million. Sales at stores open at least a year were down 19%, in part because of weak sales in the mobile phone business. For the year, the company lost about $400 million, vs. $139 million in 2012.