Season four of Bluegrass Underground is a sold-out weekend at Cumberland Caverns.Thousands of visitors are expected to converge on Warren County for four days of music in the acoustically rich confines of the Volcano Room at Cumberland Caverns. All three shows with the Bluegrass Underground series are sold out Friday, Saturday and Sunday. In addition, a special show featuring Widespread Panic is sold out Monday night.Downtown McMinnville is rolling out the welcome mat to out-of-town visitors as merchants have extended hours this weekend and bands are set to perform Friday, Saturday and Monday nights at Collins River BBQ.“Bluegrass Underground is one of the best things to happen to this town,” said Christy Ross, president of the Downtown Business Alliance.