I've just begun learning the clarinet and I have a trigger finger problem with the fourth finger of my left hand. Can anyone recommend some exercises or other therapies that would help with this?

If your finger really is triggering - flexing or extending so far, then stopping so you have to make a big effort to continue, until "ping", it moves again, I'm afraid you'll need to show it to a doctor to get it sorted. If it is more a case of a joint giving way, or buckling, then practising slowly whilst thinking about not letting it collapse may help. Good luck with it!

squeeze a tennis ball a few times a day. but not too hard.