Why 5 answers in My activity's list but only 4 in question page ?

I looked up your question, yes, you did indeed get 5 answers originally. About the missing answer: the user may have removed it himself, or it may have been reported for violating the rules and subsequently got deleted by Yahoo. In such a case the answers count doesn't get reset on the list - it's weird, but that's how it works on YA. And no, you cannot read the deleted answer anymore, it's gone for good.