Alice's most valuable clue is number 4 to Bob. This would be a Play Clue on the red 4 and initiate a Self-Finesse.
Alice can expect that Bob to play into the Self-Finesse because:
Bob will know that the number 4 clue cannot be a Save Clue. (No 4's are currently critical.)
Bob will know that the number 4 clue cannot be a direct Play Clue, since there are no immediately playable 4's.
Bob will know that the number 4 clue cannot be a Reverse Finesse, because Cathy does not have any playable cards on her Finesse Position that connect to a 4.
Alice could also clue number 3 to Bob as a Play Clue on the red 3, but this would only "get" one brand new card instead of two brand new cards.