Is it safe to still use an iPhone 4? I've received an iPhone 4 for free and I would like to use it. Last iOS version available is iOS 7.1.2 released on June 30, 2014 which is now unsupported.

My questions are: Is there some known security vulnerabilities on iOS 7.1.2 and is there known hardening techniques available.

This phone was almost not used and the hardware including the battery is almost new. It would be a shame to use it only as a clock watch.
A full list of security protection systems is available on Apples website here.

It contains the impact of the issue and what is fixed. Ultimately, it’s up to you to decide.