To help the 4th finger vibrato, experiment with supporting the 4th finger with the 3rd finger when vibrating and listen to the difference in sound.
Playing intense vibrato with 4th finger can be a challenge for several reasons:
- The 4th finger is a narrow finger and has less padding to help the sound.
- The slope of the hand means that the 4th finger touches the string at a more difficult angle for vibrato.
Compare the ease of the 2nd or 3rd finger to the 4th finger vibrato. In higher positions we will sometimes change our fingering to use the 3rd finger instead....