Depends on the 4 letters. Spells a common name, lots. Makes sense when the tld is added (fish.net), even more.
"cari.net" isn't at all bad...someone would likely buy it, maybe mid 3 figures at most...but it would require finding the right buyer...
Bear exactly correct, you always have to find the right end user. Vieri, you can always work up a site and rent the domain name (traffic), I know a few people who do that and it works out for both parties.
I have noticed 3 letter .dot coms are impossible. There is a chance for 3 letter .Net.
I could see good value in 4 letter .com's especially if their meaningful, since its the only .com's available these days. 4 letter .net in my opinion will have a lower value since there is 3 letter .nets available, if its meaningful it should have potential.