What is the difference between a 4 and 4½ digit multimeter?

A whole digit can represent digits from 0-9. A half digit can only represent a “1”, but a 2 can no longer represent a whole digit. This means a 4-digit ½ display can show values from 0 to 19999. A 4-digit display, on the other hand, can only show values from 0 to 9999.

4 ½ digits:

4 digits: