I think 4 is pretty average, but of course some are later or earlier. When DS went from pre-K to Kinder he went from 1 nap to no naps at school. It was a hard transition and he usually still napped on the weekends until his body got used to the new schedule.
I think most are around 3 to 3.5 years, but I have 20 nieces/nephews and it can range from 2 to 4. I fear DD will be on the early side since she's never been a great napper and she dropped her second nap early.
Re: Naps