iPhone 4/5 SIM sizes
The solution to your problem is, either you get it from service provider or cut it by yourself to nano sim size (as stated by other user). You might know that iPhone 4S uses micro sim whereas iPhone 5S uses a nano sim, so to use it with iPhone 4S again after cutting it, all you need is a micro to nano sim adapter which is available online for few bucks. This way you can use it on both devices.