In 3.8 I used the attachdata_postsave hook to update rows in the attachment table after an insert. In 4.0, this hook doesn't appear to be called. What's the proper replacement?
What do you mean it isn't called? Perhaps you need to change your code to use the variable names in that function. It's hard to give you a specific fix when we know nothing about your code.
Just an FYI, but attachments changed quite a bit in vB4. You will not be able to use your old queries - they will need to be modified.