Installed IOS 9.3.6 on my iPhone 4S last night through iTunes 12.9.5.5 on my Mac Pro last night.
Initially I couldn't connect to WiFi ( last night ) but today I find that the 4S is connecting to WiFi .. maybe it needed time to realise that it is now IOS 9.3.6. Caching ? No idea.
Everything seems to be working .
What really surprised is that this 4S actually seems to be faster in all operations !
I rarely need nor want to use my 4S for anything besides as a phone + LINE ( social media app popular here in Japan ) , music, photos.
The SIM is from a hobby knife hacked 3GS.
Think of the $$$$ I've saved not upgrading to iPhone 5, 6, 7 .
This 4s does everything I need to do but now a little faster ! ! ! ?
PS : The ONLY thing I've had to do since buying it for ¥5,800 = US$54.49 + 8% sales tax here in Japan in 2015 has been to replace the battery which took about 30 miutes.
Very happy with IOS 9.3.6.
IMPORTANT NOTE : Before starting the IOS 9.3.6 upgrade I set the 'Auto OFF' to Never.