Validate number of characters in a field with a message

Greets, all:

I found this formula for validating the number of charcters in a field:

event.rc = (event.value=="" || /^\d{9}$/.test(event.value));

...but in my case I want to make sure there's only four characters in the field (for the year number). It's easy enough to see that I need to replace the 9 with a 4, however, how do I code it so if four digits aren't entered for the year a warning pops up that says, "You must enter a four-digit year"?

TIA for your help!