Michigan Avenue’s dead zone is coming to life. Developers are descending on the dreary three-block stretch south of the Chicago River, still, mainly just a place tourists pass through on their way to Millennium Park or office workers go to grab lunch. Four hotels, two apartment towers and new retail space are in the works, projects that will bring more out-of-towners, residents, and stores and bridge the gap between the Magnificent Mile shopping district north of the river and the park and cultural attractions to the south.