so you need to be rooted in order to use MicroG (not sure what you mean by "signature). and in order for you to flash custom roms your phone needs to be rooted.
the main place to go for all things root is
https://forum.xda-developers.com/. it is here that developers will post guides as well as custom roms and recoveries. if your phone has a forum here, then that means that there is a community that supports your phone. unfortunately, there is no forum for the LG Stylo 4.
however if you google how to root your phone, there are plenty of sites that claim you can root the phone. they kind of look sketchy since they are not coming from xda. but as far as i can see, the LG Stylo 4 has a locked bootloader and thus can't be unlocked.....so there is no true root method. all of the sites i see are bogus. i would not trust them.
and do not try KIngoRoot or KingRoot. they have been known to install adware and have been banned from xda.
also if there is a root method you most likely will need a computer.