Sit up straight. The bow does not necessary need to travel parallel to the bridge to have a good tone. It may be more natural to be angled such that the frog is lower than the tip on a down bow, and the frog higher than the tip on an up bow. Slow bows are difficult because the body stops moving while you are bowing. Keep the body in motion at all times.
Immobility causes tension.
The left thumb doesn't necessarily want to be opposite the second finger. Let it go where it wants, depending on the passage being played. Release the thumb when playing fast notes. The 4th finger is only weak because of the standard hand position. Roll your hand as you play each finger so that the weight is transferred to each finger.