That was quick -- the judge just REVOKED Chris Brown's probation in the Rihanna beating case, but the singer will remain a free man ... for now.

Brown showed up at court today to fight the L.A. County D.A.'s motion to revoke his probation in the wake of hit-and-run charges stemming from a car accident in May ... but the judge granted the motion anyway, paving the way for a probation violation hearing in August.

If the judge determines Chris violated his probation, the singer could be sentenced to 4 years in prison.