

You can put graphene on a 4. It won’t be supported but will be more secure. Still not recommended by me or grapheneos. They say that the reason they support pixels is due to the security, not the aosp support, and that they will continue to develop grapheneos. Android 16 is the first new build with the changes and grapheneos is testing it now iirc. I would personally recommend seeing how 16 goes (possibly post after the update hits stable asking for user feedback) and decide based on that. You should probably get something new as the complete lack of security updates is quite bad but that could help you decide whether to go with graphene. Personally, I am on grapheneos and am not worried about it.
Not worrying temps for most stuff. If you have mechanical HDDs you may want to check those specifically.