Can you go over the timing for P3 and P4? Specifically, what's the difference if you're doing rounds back-to-back, or with large breaks in between?

Because of work issues, the only time of the year it really works for me to do a round is January/February. So I did my first round in 2012 and my 2nd round in 2013. I've just started P3 for my third round, and if I do another round (I'm an optimist) it won't be until January of 2016. So, how long should I do P3, and how long for P4?