Depends on your goal
If you're just looking for longterm security updates them I would recommend LineageOS
If you're looking to de-google with a privacy-focused OS then I would recommend Graphene
if the graphene leadership is a worry then calyx os and if you hate the graphene leadership so much that you want to have a source available os then copperhead os
GrapheneOS flashes fine on the 4a 5G, just did one for my wife last week. Can't say I'm happy with the theme they use, but that's a minor quibble. Keep in mind, there is no google play store on it. It's degoogled. You can set up Fdroid and then use that to install Aurora Store that will give you a bunch of the Play apps. But things that rely on Google Play Services might not work.
Very easy to convert back to factory image, there's a link in the install procedure to jump back to stock (which is all done via a Chromium based browser's USB port access).
Doesn't hurt to stick with PixelExperience, put it on an old 2XL and gave it to my sister and she uses it as her main device. Basically stock pixel (hence the name) but community updated with security patches & new releases
I have Calyxos on my 4a5g. They intend to support it until January 2025. Earlier I had Lineageos installed and was very happy with that, too, but the updates were a bit slow.
LineageOS comes to mind
I recently installed Lineage OS on my Pixel 2 and i just have one word to describe the ecperience after nearly 2 weeks: perfection.
Thanks for the tips everybody
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