As I read it, you got exactly what you said you wanted!
Since you can't expect to be paid in advance, any request to switch to payments on a 4-weekly basis is bound to leave you with a four week gap between the last payment you've received and the next (far bigger) one.
So, if you last received a payment on Tuesday of last week, your next one should be four weeks later, which would be 1st December. That's exactly what I'd expect to happen with an 'immediate' change because the computer had been told to leave that four-week gap.