A Guide to Position Designation and Investigation Type

hi Marko
i have a quick question
my background investigation for OPM is on going right now, and mine was one of those
non-sensitive public trust (high risk) no clearance tier 4
i have been searching and looking into every page i could find what this tier level 4, and i guess this is pretty new?
i tried freedom line and uscis staff and they do not know what this tier level 4 is.
i was wondering what goes through during this tier level 4 background investigation.
Thank you