Four past winners hit the scales Friday and reigning champion Joey Chestnut is the heaviest at 230 pounds. Boston gospel singer Michelle Brooks-Thompson will perform the national anthem as Keith Lockhart celebrates his 20th year as conductor for the annual Boston Pops Fireworks Spectacular on Saturday. The star-studded event is held at the DCR Hatch Cell in the city's Back Bay off the Charles River Esplanade and will also feature Broadway star Michael Cavanaugh, "American Idol" finalist Melinda Doolittle and "America's Got Talent" finalist Sons of Serendip, a quartet who met at Boston University.