The value would depend on what the domain is, whether it's a word, or an acronym or something that sounds cool .. There's no general figure for a 4 letter domain name.
I totally agree with you, but I don't think any 4 digit domain would be worth regular fee (there aren't any available).
NNN.com are much more valuable than LLL.com. I would say the general rule would apply to NNNN.com / LLLL.com also. I came along this site. I am not sure how accurate this is, but it should give you a general idea.