Why is this, you ask? While your device will technically be able to run iOS 8, the folks at ArsTechnica have done some testing and they found out that the end result might not necessarily be a good one. As you can see in the chart above, it shows that with iOS 8 installed, launching time of apps has slowed down somewhat.
Of course this is to be expected given that the iPhone 4S is three generations behind the iPhone 6. This is a phone that sport an older A5 processor clocked at 2.4GHz and is the last iPhone with a 3.5-inch display. In fact as you can see, launching Safari on iOS 8 takes almost twice as long!
However we guess there are some of you guys who will go ahead and update anyway because of the new features, but like we said this is something you should take into consideration. Alternatively if money is an issue, you could always turn to the iPhone 5c which a slightly more affordable option.