How can I add Recaptcha Version 2 for civicrm 4.3
I was able to apply these patches directly to CiviCRM 4.5.4.  Looking at the code, this version is virtually identical in the ReCAPTCHA code, so it should work fine (although you may need to manually apply the patches).

Update this file:
/civicrm/packages/recaptcha/recaptchalib.php

with:
https://github.com/civicrm/civicrm-packages/pull/92/files

Update this file:
/civicrm/CRM/Utils/ReCPATCHA.php

With:
https://github.com/civicrm/civicrm-core/pull/5015/files

In browser go to:
https://www.google.com/recaptcha and update / create a V.2 key for the website.

then go to:
(Administer -> System Settings -> Misc)
civiURL:  /civicrm/admin/setting/misc?reset=1 

Update the Public / Private keys with the new keys.

Cleanup caches: via your civi URL:  /civicrm/admin/setting/updateConfigBackend?reset=1