The most reliable way to watch YouTube on iOS 7.1.2 today is through the web browser. and go to m.youtube.com.
It is the most stable OS for this hardware. youtube for iphone 4 ios 712 download upd
For an iPhone 4 running iOS 7.1.2, you can still access YouTube using the following methods: Direct Solutions Use the Mobile Website (Recommended) : The most reliable way is to open m.youtube.com The most reliable way to watch YouTube on iOS 7