The reversed Four of Wands card shows us a situation where we are stuck and unable to make progress. Our current effort may go nowhere, so we need to take a different direction. This means you may need to revise your theories about the rainbow as a sign of luck.
The reversed Two of Swords indicates a time of confusion and uncertainty in which one does not know how to proceed. Your attitude can be negative or not very utilitarian, but also a little too optimistic. In this case, the challenge is to accept one's emotional state and use it as a starting point for formulating more precise and thoughtful theories.
Based on what we have just discussed, we can conclude that the rainbow after a sudden rain is not necessarily a sign of luck. It could mean that you are going through a difficult period that requires new perspectives and positive energy. In my opinion, the answer is therefore no: the rainbow is not automatically a sign of luck.