Assuming the claims are legitimate, it appears that in 4 days and 8 hours a hacker group will reveal "about 1000" KKK member's identities. They gained access to a website and the database of it, and through that gained the information they're going to need to pull this off. The reason for the delay is they're putting all of the information together and vetting out innocents, according to them.
Stormfronters seem to be worried that their careers are about to be ended, and I can only assume that since they've posted the addresses of people in the hopes that they get killed, they fear that the retaliation against them will be as fierce as their own retaliation to anyone who opposes them. Not the most irrational of fears, but I have an incredible amount of trouble feeling bad for them. I imagine most people do.
I view the threat as credible simply due to the fact that they've done it before. The KKK itself admitted that that incident cost them a lot of members. Rather hilariously, they called Anonymous "Scumbags" "Communists" and "Anarchists" (The 'communist' and 'anarchist' bit came in the same sentence from the same guy. Smart ones, these guys.)
Don't have much more information on this at the moment. Late at night, about to lay down, but I'm certainly looking forward to Thursday now!