Undecidability – Reduction: L1's decidability is unknown and L2 is undecidable


About this question:

Reductions can be difficult, and you want to avoid getting yourself on your knees
wrong way ”with them. In which of these scenarios does L1 ≤ m L2 provide useful information (and in these cases, what can we conclude)?

  1. L1's decidability is unknown and L2 is undecidable (nothing)
  2. L1's decidability is unknown and L2 is decidable (L1 is decidable)
  3. L1 is undecidable and L2's decidability is unknown (L2 is undecidable)
  4. L1 is decidable and L2's decidability is unknown (nothing)

in the "L1's decidability is unknown and L2 is undecidable" I know we can't say anything, but I can't find evidence of why.