many users exploit this with "tethered" tweaks. You can downgrade your 9.3.6 device to 6.1.3 (tethered), but for 9.3.6 itself, checkm8 enables Tethered Jailbreaks , not Untethered.
Tethered Jailbreak vs Untethered Jailbreak - Difference and ... - Diffen ios 9.3 6 jailbreak untethered
Why? Because the iPhone 4s on 9.3.6 is incredibly unstable. If you had an untethered jailbreak, and a bad tweak caused a bootloop, your device would be permanently bricked (restore to 9.3.6 is no longer signed by Apple). With a semi-untethered jailbreak, you can simply reboot the phone, delete the bad tweak from safe mode (via Volume Up button), and re-jailbreak. many users exploit this with "tethered" tweaks
You still need to tap an app after every reboot, but you never need to touch a PC again unless you wait 7+ days without rebooting. - Diffen Why