I whant to get snapchat at My iPhone 4

To get apps you go to the app store and download them

Snapchat by Snap, Inc.

https://itunes.apple.com/us/app/snapchat

However that app requires iOS10 so you may not be able to do it on your model of phone. You can try and see if it will let you download an older compatible version if one is still available in the app store

As Rudegar says, you can’t. Your phone simply does not have enough ‘oomph’ under the hood to run iOS10. If you must have snapchat you need a newer phone or you need to search to see if there is something else that will interface and post to that service that will work on your phone. Unfortunately that phone is just old enough and iOS7 - which is 4 versions ‘too old’ will make that something that is hard to find.

You cannot. Snapchat’s iOS app requires iOS 10 or higher and your iPhone 4 cannot be updated beyond iOS 7.1.2.

it's not possible to get ios 10 on this phone by any means