iPhone 4s reboots on unlock
First things first, always backup.


No jailbreakable iOS version is being signed so no, if you restore it you'll lose your jailbreak. 


The problem is probably a theme relates package, try disabling winterBoard themes. 

I would suggest reinstalling the system anyway because if your safe mode isn't working it means that the private system partition is broken, even if you got it working for some reason it would have loads of problems in the future, unfortunately, you'll lose your jailbreak. 

Also, I'd suggest making a backup with iMazing, a great OS X (and windows) tool to manage your iDevices in more detail. 

Sorry for that, 

Tiferrei