Since the Pixel 4 series no longer gets security updates, GrapheneOS doesn't support it anymore and discourage using it for security purposes:

https://grapheneos.org/faq#supported-devices

GrapheneOS isn't available for anything older than the 6 series.

In general graphene OS is better, but on a pixel 4a it might be that their support ends at some point soonish. I heard lineage supports devices much longer, so maybe in your case its better to check this before deciding.

Graphene is more secure than lineage, but either are way better than stock

Edit: You should really try to even get yourself off of MicroG and not require it.

definitely not LineageOS for microG

https://lineage.microg.org/

LineageOS for microG builds are way behind lineage

LineageOS for microG = lineage-21.0-20241205-microG-bramble-boot.img

https://download.lineage.microg.org/bramble/

I would use one of these;

lineage = lineage-22.2-20250823-nightly-bramble-signed.zip

https://wiki.lineageos.org/devices/bramble/

if you want to use microG use E/os

https://doc.e.foundation/devices/bramble

I have used LineageOS for microG on a Oneplus 6T and it didnt go well for me.

But lineage and E/os worked great

Your only option is lineage since it's not getting security updates the other ROMS won't support it

@tigolbitties
LineageOS for microG is a very good choice if you want to continue using your Pixel 4a 5G with up-to-date support and Google-free services. It combines the proven LineageOS base with microG, an open emulation of Google components, so you can use Google services without installing proprietary software. There are also regular monthly builds and OTA updates, which ensures stability and security. For a good balance of functionality, privacy and long-term use, LineageOS for microG is therefore highly recommended.

You can find more information on the project page: lineage.microg.org