Can I buy a sim card to put into my old iPhone?

Basically this is the situation, please read it all! I have an old iPhone 4 which I upgraded to an iPhone 5S several months ago, so basically absolutely everything on my iPhone 4 was transferred to my new iPhone 5S and after about half an hour my number changed over to my new phone and so the signal went blank on my old iPhone. I want to use this old iPhone as a 'festival phone' for reading festival, and I really need to know if it's possible to buy some kind of pay as you go sim card for this phone so I can use it for the few days I'm away at reading for? And if this is possible, how much would it cost to get the sim card? Or would it be cheaper to buy a £5 phone from Argos? (bearing in mind you still have to top up a minimum of £10, bumping that cost up to £15)

Probably cheaper to with Argos? Why? Basically without a sim inserted it is really now just an iPod touch. You can everything you did with it before on Wi-Fi, the only thing you can't do now in place calls or have cellular network connection. If you could sync and download your reading to the device prior, or might have Wi-Fi contact there, then it could work for you. Best luck.