It's called "Notes C API." The file is called CG7K2NA.EXE and was posted to the password-protected IBM Software Access Catalog on Sep. 14, 2001.
I guess the reason that you don't find it on publicly available Web sites is because Release 4.X is no longer supported. I am sorry, but I am not allowed to make the C API for Notes 4.X available to you.
My suggestion is that if you are an IBM Business partner you get access to the Partnerworld for Software Business Partner Zone and get the Value Package. This gives you access to downloads.
If you are a customer, you should go through the support channels. They ought to be able to help you.
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