Should I install iOS 8 on my iPhone 4s?
Advantages: 


more security
more features: quick reply, continuity (if you have Mac &amp;
Yosemite )

Disadvantages:
it will run slowly
you won't get a lot of the features that 5S and above have
could decrease battery life


If you have experience with ssh and jailbreaking, I would say go ahead but make sure you can downgrade using ssh just in case you don't like it. 
However, if you don't even know what that is, I wouldn't recommend upgrading because there is no way to downgrade if you don't think you'll like it because apple already stopped signing ios7 and below.