klei2002 It's still possible to install GrapheneOS on a Pixel 4, but the device is end of life and therefore only on extended support.
If you'd like to try out GrapheneOS, you could give it a go, but I would not recommend using it as a daily driver, since it can no longer be considered secure.
I would suggest replacing it with a Pixel 6 or Pixel 7 series device.
For more information about device support, please read: https://grapheneos.org/faq#device-support