The reason for the 2 techniques in the title is because I keep getting them mixed up! I need help with the technique where you left hand fingers go up and down on a string rapidly...I think it`s trill...

Anyway, does anyone have any tips on how to get faster with this technique? My first, second and third fingers seem ok, but my forth finger is the slowest and it seems to `freeze` if I have to do this technique with that finger.

Also, anyone know how to make the 4th finger stronger (or any in fact)...e.g. so that when I press down on a string in 1st position it stays down and hits the fingerboard?

Thanks

Trill: \/\ or \/\/\ or tr or tr\/\/\/\/\...
Tremolo: Rapidly alternating bow direction

There`s a book for left hand exercises called Schradieck. I don`t think I spelled it right. It`s good for developing a dexterous left hand.
Practice the trills in short sessions (about 5 minutes or so), many times a day. Sorry I`m not much help.

Practice curving, lifting and throwing the finger 4 on the fingerboard. Make sure u can hear the `thump`! with each note. Practice this every day and make it go faster, and lastly incorporate it in whatever ex u have.