What does a half-digit means in case of accuracy?
3 digits would be 0 through 999
3 1/2 digits is 0 through 1999 (typical for DMMs)
3 3/4 digits is typically 0 through 3999  

Has nothing to do with binary digits, but decimal digits, or rather their representation in 7-segments displays. To display every digit you need all 7 segments, but if for the fourth digit you only have to display a "1" you only need the two rightmost segments, so that can be interpreted as the right half. That was when most DMMs had a maximum reading of 1999. Recently more accurate DMMs became available, having readings up to 3999. If "1" as the highest value for the highest order digit is half a digit, with some imagination you could say that a highest value of "3" is 3/4 of a digit.  

Note that for displaying only "1", "2" and "3", you don't need the upper left segment, which a 3 3/4 digit DMM indeed doesn't have for the leftmost digit. It's a small cost saving, but a saving nevertheless.