Sentence 4 initiates an inquiry into why certain crimes hold a lasting allure and why some cases are repeatedly revisited.
Sentence 1 expands on this inquiry, delving into the specific "unusual elements" that capture the interest of a particular audience, narrowing the focus.
Sentence 2 proposes potential factors, such as extreme depravity or enigmatic circumstances, that contribute to the appeal of these cases.
Sentence 3 then broadens the concept, suggesting that unresolved cases, which may defy solution, offer a distinct appeal absent in "ordinary" murder cases, highlighting the enduring fascination with unsolved mysteries within the realm of crime.
So, the correct option is (A): 4123