How to tell whether my (UK) iPhone 4s is 4G ready?

In a couple of months I reach the end of my contract with Vodafone, and I want to keep my handset, but switch to a 4G tariff with EE. My phone picked up 4G in New York, but how can I tell whether it is equipped for 4G in the UK before I buy a 4G sim card?

You can't make the iPhone 4s 4g compatible as it does not have the LTE antenna like the 5 does. Your only option is get a LTE phone or forget 4g.

Your phone is not capable of receiving 4G
your phone was made before 4G was even launched

The iPhone 4S is only capable of 3.5G/HSDPA+ data speeds at best (these are branded as 4G in the United States).