4chan captcha key:
6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc

Google supports a noscript version of the new recaptcha, described here https://developers.google.com/recaptcha/docs/faq

https://www.google.com/recaptcha/api/fallback?k=6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc
Put this in an iframe and when the user has successfully filled in the captcha the user is told to copy paste the key into the real website field (outside the iframe, g-captcha-response)

https://www.google.com/recaptcha/api/challenge?k=6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc
contains the key 'challenge'

Now load the image
https://www.google.com/recaptcha/api2/payload?c=CHALLENGE

after the user has solved the image, do a POST to

https://www.google.com/recaptcha/api/fallback?k=6Ldp2bsSAAAAAAJ5uyx_lx34lJeEpTLVkP5k04qc
POST:
c = CHALLENGE
response = USER_RESPONSE

You'll have to get hash inside the textarea of div.fbc-verification-token

next send off the reply to 4chan
POST:
g-captcha-response: HASH