Landau's four problems are now over a century old (1912), and each still unsolved. This seems remarkable, even though he was not the originating author all four (maybe only the 4th?). Still, he isolated and listed them as challenges.

Of course Hilbert's $23$ problems (1900) have been hugely influential, but many have been resolved in some form; perhaps $4$ sharply defined problems remain completely unresolved. William Thurston's more focussed $24$ problems (1982) are largely resolved: Thurston's 24 questions: All settled?. Steve Smale's 18 problems (1998) are perhaps half solved.

What other such lists have mathematicians publicized? Is there any single researcher's list comparable to Landau's in duration remaining open?