Here's a partial list I put together based on my own experience, I don't know how correct it is. The IV was a massive upgrade adding daylight savings time, improved accuracy, a second timezone, a stopwatch, and an inaudible, visible-only alarm. However, the IV did not have the AM/PM indicator of the III. The IV also used what it called a "jog" (like a doubleclick) for accessing or setting some of the new features. I'm not 100% certain, but I think the III didn't have this.
SYNCHRONAR 2100 (Mark III): Riehl Time Corporation
* Features: Hours:Minutes (24 hour; or 12 hour with AM/PM indicator which appears as a single LED element in the display);
* Date;
* Day:Seconds;
* Leap Year;
* Rate Adjustment 16 seconds per year increments (+/- 8 secs/year accuracy)
2 slide switches
SYNCHRONAR 2100 (Mark IV): Custom Circuits Corporation
* Features: Two time zones (Hours:Minutes;seconds) (24 hour or 12 hour - AM/PM indicator feature removed);
* Date;
* Day:Seconds;
* Leap Year;
* Rate Adjustment 8 seconds per year increments (+/- 4 secs/year accuracy);
* Automatic Daylight Savings Time Adjustment;
* Alarm;
* Stopwatch;
* Flashing timer
2 slide switches