I'm not sure if you're asking about downgrading to iOS 6 or upgrading to iOS 9.
I think the 4s runs iOS 7 very well. Sure its slower than 6, but it is still fast and smooth on iOS 7.
You will notice a speed downgrade to iOS 8.4.1 (which is only possible via Odysseus OTA 2) and a speed downgrade to iOS 9
iOS 8.4.1 is faster at launching apps and typing on keyboards than iOS 9, but iOS 9 is smoother in terms of animation lag and multi task switcher
If you downgrade to iOS 6 (only possible via Odysseus OTA 1) you will notice a speed increase (4s is very fast and smooth on iOS 6 and launches apps instantly) but you will lose FaceTime and many third party apps do not support iOS 6 (even less than those supporting iOS 7)
But if all you use your phone is for texts, checking mail and phone calls iOS 6 will give you the smoothest performance
iOS 7 is perfectly fine on iPhone 4s
iOS 8.4.1 will give you more app compatibility and new features which I love personally such as quick reply texting, but it will noticeably slow your phone down (note its about 2x slower than iOS 7 and much more animation lag/stuttering)
iOS 9 will further slow the phone down a bit, but its more or less the same as iOS 8 right now. It still launches some apps slower and the biggest weakness is typing is not responsive which is the "achilles heel" of the 4s on iOS 9. The big positive it has over iOS 8 is that its smoother. If Apple can fix the keyboard and typing lag, and get apps to launch closer to iOS 8 speeds all across the board, it will be superior to iOS 8
But be warned, once you leave iOS 7 you cannot go back to it. It is also much faster than iOS 8 or 9 on the 4s, though iOS 6 will give you the best possible performance.
I don't know about waze and 99taxis but google maps works perfectly fine on iOS 6, and of course all of those apps should work on iOS 9
