The one song/4 minute limit may work for someone who needs no set-up time, but typically it takes 5 minutes at the very least to get one performer unplugged and off the stage and the next performer up onto the stage and situated and preliminary sound levels checked. Keeping the performer on-stage for at least two songs means a higher ratio of music (no matter what quality of music) vs. emcee-chatter filling the empty air between performances. And more music means more audience interest!