It’s an incredible iPod touch. Use it for music (Spotify legacy or local MP3s), podcasts, and reading ebooks. For YouTube, consider it an occasional treat, not a daily driver.
Although there are no official ways to download and install YouTube on an iPhone 4 iOS 7.1.2, there are some workarounds you can try: youtube for iphone 4 ios 7.1.2 download
Even with the best methods, you may encounter errors. Here’s how to fix them: It’s an incredible iPod touch
Disclaimer: This article is for educational purposes. Downloading modified IPAs or jailbreaking may violate terms of service. Proceed at your own risk. and reading ebooks. For YouTube
If you're still willing to take the risk and download YouTube on your iPhone 4 iOS 7.1.2, here's a step-by-step guide: