4 year old biting her nails – Yikes!

A few weeks ago, my 4 year old started biting her fingernails. At first I stared at disbelief – isn’t this supposed to be something that only grown-ups do, much like driving and making responsible decisions…but no! She was really doing it, and with a zeal too!
I asked other parents from her school and the playground, consulted with friends and family, and was only mildly comforted to learn that this is a very common behavior in children.
In terms of (failed) attempts so far, we have tried the following: verbal corrections, pep talks, rewards (she loves to go to the nail salon and get nail polish, so my wife and I promised her that we would take her there if her fingernails grow long enough), and even a special bitter nail-polish that is supposed to remind her that this is really a disgusting habbit. The latter attempt was by far the funniest – as she genuinely got excited about the bitter nail polish, but soon realized that biting her nails will earn her another application, which she loved!
We never experienced anything like this with her 7 year old sister, and would really appreciate any advice from any of our many readers!
Regards,
Parents of 2 girls