Auto on headlights?

We don't even have the "OFF" position on the stalk here on the UK Prius. The default is "AUTO" for auto on and off. No chance of operating in stealth mode at night.

 

It's like many things on cars.

You buy the top PREMIUM model and get things which they've purposely put on only that model. The rest of us just dream about those things. (Or didn't want them anyway - like heated seats which aren't needed in the tropics.)

It's about encouraging people at purchase to spend the extra $$$.

Same happens with most things in life - you buy a 65" TV and when you get it home wish it was 70" - but it'll never be 70" no matter what fertiliser you use.

 

For what it's worth, there was no coding change needed in the Body ECU on my 2010 Gen 3; as soon as I completed the missing wire run from it to the solar sensor, it started happily controlling the headlights.

That may tell you absolutely nothing about Gen 4, of course.