I would be perfectly happy with £25 on a 4 weekly.
Why not give them the option of every-other-month (or eight weeks; whateva) and charge 'em £35.00, pointing out that it would work out cheaper.
Every-four-weeks is too regular, IMO; it's like 'Is it that time again already?'
I totally understand where your coming from, sometimes i have to get my book out and check dates before i start in an area.
However, since i officially begun in 2003, i have always cleaned every four weeks and this simply continued when i switched last yr. I sell it to new customers that apart from bird mess, there windows will remain clean for duration of four weeks till next visit.
If i change all my work over to 8 wks, im cutting my income by half so in short term this is no viable solution. So 35 every 8wks compared to 50,,,,,erm no thank you. Not till my customers start moaning at me being on time, then i might consider it.