What is the upper and lower bound of 4.5?

When you right 4.5 we assume that you were reading a scale marked in units, and you estimated the decimal. So the exact reading could have been between 4.45 and 4.55 If you say 4.50 we assume the scale was marked in tenths and you estimated the hundredths position. And so on.

Some scales are not read that way. For instance a digital meter is never certain about the first digit, regardless of the claimed accuracy.