Does the iPhone 4 get Snapchat's faces update?
Yes, the update should work on your iPhone 4 as long as it meets the requirements: iOS 7.0 or later. Compatible with iPhone, iPad, and iPod touch. You need to go to Settings > General > Software Update and apply the latest updates. After this procedure, install Snapchat using the App Store's link and that's it.
App Store's Snapchat link: https://itunes.apple.com/us/app/snapchat/id447188370?mt=8