Explaining the unexplained rooting terminology? LG G4
Those are just build numbers of stock ROMs you can see in Settings - About Phone. Letters usually indicate minor updates and numbers major updates. By major it doesn't necessarily mean Android version bump - it could patch a number of important exploits, often including ones crucial to rooting.

A tool will be released with the exploit suitable for a version and all versions below it. Therefore, as long as the tool claims to root that version, you can do a lower one just fine.