I have the Oasis 3, and have seen the overheating warning many many times - I didn’t know people didn’t know about it.
I live in South Texas, and frequently read outside in the summer, the temperature is often between 100 and 110 F. (I’m in the shade or in the pool, not directly in the sun, by the way). Sometimes I’ll go back inside to get a cool drink and the warning is on. I see the warning without the cover on, FYI.
I also have a white plastic Kobo Libra H2O and have never experienced any overheating problems with this. I used to have a Kobo Aura One which I used in the same conditions with no problems.
My other half has used the Paperwhite 4 many times in the hot sun, with no issues, I’m inclined to think it will be the same for the new one. That metal back is a killer - just what I don’t want in an ereader - scalding hot in the summer, and freezing in the winter. (Or the two days here when it drops to 32
).