Frontier Airlines will be adding four new flights from Phoenix this week. Beginning April 14, the low-cost airline, which is owned by Phoenix-based Indigo Partners, will begin offering nonstop service from Phoenix to Atlanta, Detroit, Cleveland and Cincinnati. The launch is part of a Frontier initiative to resume and increase flights in 40 markets this week. Frontier Airlines had a 34 percent decrease in passenger traffic in January — the latest data available — compared to the same month…