Computer braille readers and most braille displays have four rows instead of three because of the following reasons.

A braille display must be able to show the cursor for editing and to facillitate keeping track of where you are. Also, certain symbols such as the carriage return in ascii text can be displayed as the letter m with a dot 8 under it. There are also special arrangements used for mathematics. I'd personally like to strangle the person who came up with it, what his name Nemeth I can't stand that particular form of braille. But for example, a fraction would look like this, assuming that you have looked at the table above.

however, when reading a recipe, ninety percent of the time the fraction symbols are omitted, except for the slash. Also, certain contractions follow a higharchy. i.e fever is not done as f-dot5-e, because it just isn't. You type fev and then use the er contraction. I've always wondered about that until I asked my vision teacher in the fifth grade, and he explained it to me. If anyone has any questions, feel free to pm me.