Bug overview description.
Uploads from is.4chan.org, which 4chan is currently using to host images, are not permitted despite 4chan being whitelisted.

What part(s) of the site page(s) are affected?
The upload page.

What is the expected behavior?
"Uploads from is.4chan.org are permitted!" message, followed by successful upload

What actual behavior is given instead?
"Uploads from is.4chan.org are not permitted. (View whitelisted domains)" message, followed by error when trying to upload

Time of incident (if applicable).
12/28/2016

Can you reproduce the bug every time?
Yes

What steps did you take to replicate this bug?
I attempted to upload post #1091012 from a 4chan thread, before eventually just uploading from a booru. However, I did not attempt to replicate it with a different URL, as I could not find other suitable images on 4chan I wanted to upload at the moment.

Errors or other messages returned (if any).
See above

Maybe they only whitelisted i.4cdn

It looks like i.4cdn.org is the only whitelisted domain, is.4chan.org would require separate whitelisting. I'm not sure if the thread is still used, but you can suggest domain whitelisting on forum #181740.