Young Thug's New Mug Shot in Drug Arrest

Young Thug is still sitting in jail, FOUR days after a marijuana possession bust that was prompted by tinted windows ... TMZ has learned.

We just got Thug's latest mug shot from his arrest Saturday in Brookhaven, GA. As we first reported ... Thug is facing a felony drug possession charge ... and now we know how much.

Law enforcement tells us he allegedly had 2.5 oz of weed on him, and cops found the stash during a routine traffic stop. We're told Thug was initially pulled over for over-tinted windows.

As for why he's still in jail ... the case is still being reviewed by the D.A., and no bail's been set.