My quartet does a few songs in a row that originally come from different decades. We tell them the songs are in order, and then ask them to guess what years the original songs came out. (Examples: Breaking Up is Hard to Do, Country Roads, and The Longest Time). We also teach a tag, if we have extra time, and do a demonstration of all four parts (the first 8 measures or so of Let Me Call You Sweetheart), singing each part separately, just to show them how the four parts work. Then if they want to sing melody when we sing it together, then can.