I do per calendar month only. The reasons for this are as follows:
1, I like to keep everything simple, keeping everything on the same regularity does just that.
2, Doing work per 4 weeks means customers are going to expect you every 4th wednesday or whatever. Per calendar month, unless i work 7 days per week, means that the customer will not be expecting me on a particular day, giving me a bit of flexibility for days where the weather is crap, or i need to take a day off for some reason.
3, Due to the flexibility mentioned, if i want to take a week, or even up to 10 days off, i can, weather permitting. I decide where i want my 'gap', then over the course of a few months, i do the stuff beforehand a bit earlier, and the stuff after, a bit later. I realise that eventually, due to volume of work building up, i will not be able to do this as frequently or as easily as i do now, but for the time being, it works a treat.
4, Again due to the 2 previously mentioned points, in winter, i never miss out parts of my round. If i get pushed back, it is not a problem, i just start back when i can, and crack on. My customers are ok with this, and when it has happened in the past, it has never taken too long to get back on track.