Pixel 4 will not receive Android 14. They're legacy extended support and are being phased out. They're insecure no matter what updates they receive, as not everything can be provided past their end-of-life date. What they do receive from GrapheneOS is a harm reduction measure to help people as they transition to supported devices.
The last device that will be receiving extended support is the Google Pixel 5a. 6th gen and up with 5 and 7 years of support since launch (7 years since launch for 8th gen +) is enough that no extended support will be required.