A study of customer satisfaction at the Big 4 wireless carriers finds no significant difference between them. That's because of improvements in customer service at Sprint Nextel Corp. and AT&T Inc. The American Customer Satisfaction Index puts Sprint, AT&T, T-Mobile USA and Verizon Wireless within 2 points of each other on a 100-point scale of customer satisfaction. That's the smallest spread since the survey started looking at all four companies in 2005. It's also within the margin of error at plus or minus three points. Last year, AT&T clearly trailed the pack, while Sprint and Verizon led. That was a surprising development for Sprint, which was last as recently as four years ago. Sprint CEO Dan Hesse has made improvement in customer service central to his tenure.