Four Minute Warning?

[GPC2]

In my game on Friday night, our clock operator never showed up. I made the decision to just keep the clock on the field myself (I was the R). I kept the coaches abreast of the clock situation during breaks in play and time outs. With four minutes remaining in the half, I stopped the clock and gave a four-minute warning.

After I got home and spoke to one of the veteran officials about the game, he told me that I didn't need to give a four-minute warning. When I looked in our mechanics manual, there was something that discussed the clock operator keeping the time on the field. It stated that inside of five minutes, the clock operator should notify the R and he should then float between sidelines keeping the benches notified of time remaining.

My question is - does anyone know of a mechanic of giving a four-minute warning when no stadium clock is being used? And also, is there a mechanic if there is no clock operator and the R is keeping the time?