6mm² would be an overkill in my opinion and the pricing are usually far different. For 4mm² if you are getting a proper price from a proper electrician the price different is not far to consider - taking account into some users who frequently use an aircon in a way that does not allow the compressor to cut off, in fact a 4mm² would present a lower-cost setup in the long run. It depends on how you calculate as some people might decide to sell a house after 5 years so investing slightttttly further does not make sense.

4mm is not really "overspeccing" if you are considering that ST rules for all the developers are now to go for 4mm².
To be exact in finding the best you would have to look into the entire setup - some older air conditioning systems for 1hp would consumes about almost a 2hp similar power, and some 2hp would consumes about 3hp like today's aircon system.

ST rule on the wire size is always "minimum" or the word "minima" so I would beg to differ that 4mm² is overspeccing, but 6mm² would be an overkill in my opinion.
Glad you took the effort to look for my old comment  - yes throughout these few months I had decided to do a lot of learnings because I need to govern how my house wiring is setup and even doing inspection in my existing and other older houses which I stayed. These knowledge also helped me to argue with many different wiremen (when I decided to approach them to do re-setup some areas which I felt can potentially be hazardous) - 9 out of 10 wiremen insisted 10mA water heater RCCB is bullshit, suggested nothing lower than 100ma  sensitivity. 7 of them had no idea what is RCBO.  Was glad I learned a lot from CEO and revisited some of her sharing time after time to calibrate my understanding of what I read which were all true.

As I mentioned, the key word by "ST" is minima and you need to factoring into the economical and supply chain issue when ST are spec-cing all these so it is up to you to find the "optimum" setup which suits your usage - Minimum vs Optimum vs Overkill. A good example is a 100mA RCCB is still allowed in Malaysia where as in Singapore standard is a maximum 30mA only - is 30mA sensitivity an overspeccing?

Anyway, I would say most people visiting the forums would benefit from knowing the "must do" and "good to do" and helped them to be able to explore more, and then making an informed decision. This makes the forum an interesting place to share and learn from each other. I benefitted from it and we all should together.