It is possible for Alice to give a Play Clue to the purple 4 either by cluing purple or by cluing number 4. Both clues would focus the purple 4.
However, this would signal a Prompt to Cathy, and she would play her purple cards from left-to-right. That means that the purple 3 would misplay from slot 4.
Thus, Alice is not allowed to give a Play Clue to the purple 4.
Instead, Alice can clue number 3 to Cathy. This would focus the purple 3 as a Self-Prompt.
After that, Alice can give a Play Clue to the purple 4 without any problems.
Note that normally, players are not supposed to give Tempo Clues, because they do not meet Minimum Clue Value Principle. But here, Alice has a very good reason to give a Tempo Clue - Cathy's cards are "out of order". So, it is okay to break the rule.