Vibrato. (for hesitant fourth finger)

Bach. (for timidity staking out shift to fourth finger in fourth position (aka sixth position))

3 and 4 Octave Scales- Kaboff ‘Ghost Note’ method. (for timid fourth finger)

Amanda Bailey’s Finger Twister (for slow fourth finger)

Starker’s Organized Method (for clumsy fourth finger)

The Bach has really helped.   The fourth finger learns from the ear, just as the ear responds to the fourth finger.  And the ear *listens* to Bach differently than any ol’ drill or etude.   I also read that visualizing the vibrato process correctly is vital.  It is tempting to think of vibrato as ‘smudging’ the pitch, but this image gives the finger the wrong idea.  The finger needs to think about isolating the exact center of the correct pitch, like a node, and anchoring it with absolute certainty to the fingerboard.  Somehow this latter image leads to a much more confident vibrato with a much more centered and cellistic sound.  Think slow mvt of Dvorak, first note, D.  Kind of warm yet kind of piercing.