disp.timerIsr(); //this seems to be the poorly-named function which actually outputs 4 characters to the display
I could not agree with you more. Particularly since the sketch is coupled with the Timer1 library. It was difficult for me to wrap my head around the timer1 interrupt ISR calling something named disp.timerISR().
If it were called disp.refresh() and was called every 1016 microseconds(or at whatever interval is needed by the particular display) by the Timer1 isr it would be more comprehensible.