If i am wrong please let me correct with giving proper explanation.
Yes, Correct! but a string can not ambiguous. Though given grammar is ambiguous!
A grammar will be called ambiguous if can generate more than one parse tree/Left most derivations/Right most derivations for same string.
One more point to be noticed here is that, in first parse tree, left id*id will be evaluated first while in second parse tree id+id will be evaluated first,
Means 2&4 both options are correct