CHRIS BROWN TO FACE FOUR YEAR JAIL TERM AFTER PUNCHING A STRANGER OUTSIDE A 5-STAR HOTEL!!!

Chris Brown was arrested in Washington D.C. Yesterday morning for FELONY assault after allegedly attacking a man outside a hotel ... TMZ has learned.

Law enforcement sources tell 

TMZ 

... Brown got into an argument with a man outside the W Hotel on  K Street and allegedly punched him once 

IN THE FACE.

   We're told the man Chris struck never threw a punch.  Sources say Chris and the alleged victim did NOT know each other. 

The incident occurred at about 4:30 AM.

Chris was with his 

bodyguard

, and we're told he was also arrested.

Our law enforcement sources say Brown did not appear to be under the influence of anything at the time of the incident.  

Brown is currently in custody.  As for the other person, we're told he's being evaluated at a D.C. hospital.  Sources say he's claiming injury.

Brown is still on probation in the 

Rihanna

 beating case and this could be grounds for violating his probation -- which means he could be sentenced to prison for nearly 4 years ... worst case scenario for him.

Before the incident, Chris had been hosting a party (pic below) at the nearby Park and 14th nightclub.