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I responded to the post here also.
The 404 would most likely indicate that the redirect url you are going to isn't available. Can you get to your redirect url normally through your browser?
It's worth mentioning that the redirect URI that you specify in the config.php file has to match what is stored in Constant Contact, which you can view by going to this link, and logging in to view the details of your API Key.
API Support Specialist
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