Why can't I update my iPhone 4 when there's no service?

Kay so my g-ma gave me her old phone (iPhone 4) and there's no service so I'm iSight it like a iPod. I wondering why can't I update it because I downloaded snapshot instagram and Facebook. The Facebook works but the instagram and snapchat says I need to update my phone before using it.so I went to the settings and usual where it says about, update and usage update isn't there it's just usage and about.so I'm wondering if there's a way I could update it?

Go on Wi-Fi

Not enough space in the phone to update and also it don't have to be on Wi-Fi all you need is to update your phone or the app

You said iPhone 4 not 4s. It will update just like an iPod on Wi-Fi, but there's a problem. Go Settings > General > About > See under Version. If it reads 7.1.X something then that's it. The 4 doesn't get iOS8. Only 7's. If it has 7 anything it's updated as far as it will get. Best luck.