i think he makes them up as he goes along and back in previous seasons can't remember which when he was reading after rule four he said there is no rule five probebly because he couldn't think of one to go with the number 5
Either it is as it says there is no rule number five or we haven't heard it yet. Maybe Gibbs doesn't like the number five. Or he hasn't made one up. In the seventh season finale he didn't seem to number them in order it was which one was most important and beneficial at the time. The most important ones seem to be the higher numbers.
Rules of Gibbs proves that Gibbs changes rules without telling anyone. Rules of Gibbs states there is no Rule #5, but NCIS Gibbs Rules states that Rule #5 is “You don’t waste good” which happens if “There is no Rule#5”were true. But because there is a Rule #5 “Rules of Gibbs” is wrong and Gibbs didn’t tell you.