I haven’t used the AHD4 over at bartleby (bartleby.com/61) for a long time because of their irritating pop-up ads.  But I just tried today and it seems to be gone.  And a quick google of “American Heritage Dictionary” produced no results except places to buy the paper copy.

I have a vague sense that someone here has mentioned the disappearance, but can’t find it.

well, if I had looked!

I presume that the reason for its disappearance is contractual or financial. Bartleby didn’t want to pay for continued use, AH raised its rates, or something like that. In addition to Dictionary.com, it is also still offered by Yahoo! Education.