Lindsay Lohan Hits NYC Bar Scene ... 4 Days Before Rehab

With less than a week to go until her court-ordered stint in rehab, Lindsay Lohan hit the NYC bar scene Monday night ... AND ACTUALLY BEHAVED HERSELF!!!

TMZ has learned ... Lohan spent her Monday evening at The Bitter End -- the piano bar credited with giving Lady Gaga her start back in the day.

Sources at the bar tell us ... Lohan rolled in to the place with 9 or 10 friends to watch some of her pals perform at a "Jam Fest" event.

We're told Lindsay sat quietly at her table for roughly 3 hours and didn't suck down one drop of alcohol -- just soda.

One source inside the bar tells us Lohan was so "very well behaved" ... people in the joint barely even noticed the actress.

As for Lindsay, she's expected to enter a treatment center in the Hamptons on May 2 -- where she's supposed to complete a 90-day program.