Many violists simply avoid using their fourth finger whenever possible. Luckily, by using this tip which you'll see in use by every great violist on the planet, you don't have to change anything:
Support the fourth finger with your third finger.
It's that simple. Practice vibrating both at once. The smallest knuckles of both fingers should bend back and forth while you vibrate. It's often even helpful for the fingers to touch, for extra support. The picture below illustrates an ideal example of how your hand should look.