iPhone 4 (iOS 7.x.x) - Unable to make video calls from any app other than FaceTime
Although WhatsApp is compatible with iOS 7, the video calling feature requires iOS 8 or later.

FaceTime is available only for Apple's iOS and macOS. You can make a FaceTime call to your contacts using iPhone/iPad (iOS 7 and later) or Mac (OS X 10.9.2 or later). It is not available for other mobile or desktop platforms.

There are various other popular cross platform messaging apps with support for video calling (Viber, Hangouts, Google Duo, Skype) but they all require iOS 9 or later to be installed.

You can give Zoom a try. It claims to support iOS 7 and later.