I have been looking for a SIM card for my iPhone 5 recently. In doing this I have realised that iPhone 5 users are limited in their choice of providers.
In the UK, maybe providers have just put their foot down and...
As far as I am aware the iPhone 4S is not 4G compatible. Only Apple's iphone 5 is. The iPhone 5 takes what is called a Nano SIM and the 4S takes a slightly larger Micro SIM.