For the Pixel 4 and 5 series, the GrapheneOS team aims to provide extended support until the next Android version. So Pixel 4 will be losing extended support when Android 14 is released, if not before. Note that the Pixel 4 is already an insecure device, the updates are just provided to give users a little time to move to a recommended device (currently all Pixel 6 and 7 series devices).
Pixel 6 and 7 series will not receive extended support, since they have 5 years of full support anyway.