I see some people saying that the IPhone 4s is now useless in 2016. but I don’t have any issues with it. it is fast, although it does lag a little bit in the IOS 9. if you want to increase the performance you can turn off the " Hey Siri" and the spotlight search.
I had the IPhone 4s for quite a while now and it is still a great phone. it is very cheap now days. so I would recommend you getting it. Although it doesn’t have the night shift, and some other IOS 9 features it is a good phone.