The Pixel 4a(5G) is the oldest Pixel model eligible to receive Android 14
You’ll see a small photo of your Pixel device. Underneath it is a box that says Opt in. Click on it and you will receive an update to install that contains Android 14 Beta 1. After opting in, go to Settings > System > System update and follow the directions. If you are a member of the QPR3 Beta program, you will first have to opt out of that program. If you opt out of the QPR3 Beta program and immediately opt in to the Android 14 Beta program, you will not have to wipe your device with the first Beta release only.
We should learn much more about Android 14 on May 10 when Google holds it I/O 2023 developer conference.