What does it mean that a multimeter is four and a half digit?
At most ranges, the smallest reading is 1/19,999 of the maximum (e.g. on the 20 volt range, values range from 2.000 to 19.999 in steps of 0.001).  Although it may seem odd to regard a meter that can read up to 19,999 as being a full "half digit" better than one which can read up to 9,999, common terminology for many decades has been to use the term "1/2 digit" to mean a leading digit that's zero [blank] or one, "2/3 digit" to mean 0, 1, or 2, and "3/4 digit" to mean 0, 1, 2, or 3.  The fraction is explained thus: the numerator is the maximum display value for the MSB - '1' or '1/2', '3' for '3/4'; the denominator is the total number of possible display values '0, 1' (hence '2' for '1/2'), '0, 1, 2, 3' (hence '4' for '3/4').  Thus, "3 3/4 digit" doesn't mean that the numerical significance is 3.75 times as much as much as for a 1-digit meter, but instead that there are three full digits, plus a digit that shows a value 0-3.

Source: What's a half digit anyway?