I beleive the 1/2 signifies the first digit having a range of 0 to 1, the 3 or 4 signifies the remaining 3 or 4 digits have a range of 0 to 9. Thus a 3 1/2 digit display has a full range reading of 1999 while a 4 1/2 digit display has a full range reading of 19999. Options include a potential decimal point that could be located anywhere, along with a +/- prefix.
Hope this helps
Wheels within wheels / In a spiral array
A pattern so grand / And complex
Time after time / We lose sight of the way
Our causes can't see / Their effects.