on Pixel 4a

fragolone

by installing GOS on Pixel 4a will I receive Android 14 or 15?

No, and you shouldn't use a Pixel 4a anymore because it's an insecure end-of-life device. Firmware and driver security patches aren't available even if you switch to GrapheneOS. Switching to GrapheneOS will provide you with the Android Security Bulletin backports and the GrapheneOS privacy/security features from around a year ago. It's a separate legacy extended support release branch for 4th generation devices. It's not going to be continued indefinitely and will end at some point.

Is there any care I need to take to install GOS on Pixel 4a since it is legacy?

No, it's the regular install process but after you've installed it you'll have the GrapheneOS UI and features from around a year ago rather than today. That means the post-install documentation and other information on our site isn't fully applicable since these devices are on the legacy Android 13 branch of GrapheneOS. We didn't port them to Android 14 since it would have caused major regressions. People are also meant to be stopping their usage of these legacy devices as soon as possible. We have to stop doing releases at some point, partly because our releases are encouraging a subset of people to continue using insecure devices who would get a new device if we stopped doing the releases. Our current plan is to end legacy extended support when the devices are 5 years old. That's getting close for the Pixel 4 and 4 XL. Pixel 4a still has around a year before that. We may still provide 4 and 4 XL extended support until the 4a is going to be dropped though.