Time to upgrade from iphone 4s?

I currently have an iphone 4s which works fine but i was wondering if its time to upgrade or not? Also i have ios 7.1.2 on it as i heard ios 8 didn't work that well on the 4s so i also wanted to ask whether ios 9 works better on the 4s?

It should.

A more worth while upgrade at this point though would be of the iPhone itself.

Ios 9 sucks on 4s