if the sim does not fit, then order your micro sim here:https://giffgaff.com/orders/mgm
sim swap, please see here:http://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084
this enables you to keep your number (and credit)
In general your current SIM should work fine in your new phone! The only exception is if the SIM needs to be a different size. Given that you are buying an iPhone 4s, I think they take Micro SIMs. If you are using a normal SIM at the moment (from what I can make out from the online manual, you should be), just pop out the middle section of the SIM and use that in your iPhone.
If your SIM does not have a middle pop-out section (or if I have the SIM types wrong), you will have to do a SIM swap. You should never have to create a new account and transfer your number, at any rate.
SIM Swap (if needed):
"SIM Swap" lets you change your SIM, while keeping your account, credit, goodybag and any other information the same. If you already have a spare giffgaff SIM the right size lying around, you just need to activate it, but this time log in to your current giffgaff account instead of making a new one. It will ask you if you want to swap your SIM and deactivate the old one, so check the SIM codes are correct and click yes.
The SIM Swap should happen very quickly (in around 10 minutes), and you will be up and running in no time.
If you do not have a spare SIM, just order a new one at the right size at https://giffgaff.com/orders/mgm.
If you want more information or more detailed instructions about SIM Swaps, visit this page:
I hope this helps!