Re: how to keep my same number on a pay as u go iphone from 4 to 5
All you need to do is just swap out sim cards, surely.
If you are going from one sim size to a smaller one, your provider can swap out the sim for you to a smaller one. I did that when going from a borrowed iPhone 4s on AT&T to my UK iPhone 6.
If, however, you are swapping carriers too then I guess you can negotiate with them to keep the same number. I honestly can't see it as a problem. They do it here in the UK for PAYG numbers and I have no reason to think anywhere else would be any different.