also, monthly tends to fit better with peoples outgoings in my area. they all get paid monthly, and only want a window cleaner 'hit' once a month.
If you do the maths as well, if you say monthly, thats 12 a year, every 5 weeks is only 10 a year! by saying 4 weeks its percieved as 'less than a month' and can be frowned upon, although thats only 13 a year!
Monthly, and i then allow for a 'swing period' in my months planning for inclement weather.