This is an interesting question.
The pink validator in this case is designed to demonstrate that you have avoided zones 1-2. If you are simply making that journey there and back then there is a saving to be made by touching the pink validator. However, if you have already used zones 1-4 and capped then touching the pink validator at Rayners Lane will charge you for going into zone 5.
The rules say that you should touch a pink reader if you pass one to ensure that you pay the correct fare. The only penalty for not touching one is that you will be charged the higher fare. In your case you are happy to pay the zone 1-4 fare because you have already capped. I don’t see a problem ignoring the pink validators in this instance.