What's going on?
4chan appears to use an invalid certificate when using HTTPS and it won't let me post.
Your AV software sucks or you're not using https, maybe a MitM attack.
What is your 4chan certificate signature?
Sha1 is BF:1D:1F:A6:F6:5D:47:66:B3:33:B0:FC:D5:92:53:8F:EF:42:34:F8 here.
whoever manages the server has to set it to default to HTTPS for automatic HTTPS, 4chan defaults to HTTP. If you want it to be default to HTTPS, get an addon called HTTPS Everywhere.