Pixel 4a is likely to stop having full security support in August 2023. (Source) After that it will increasingly be insecure to use the phone, as security exploits won't be patched. This is antithetical to the goals of GrapheneOS.
It is probably more economical to buy a Pixel 6, 6a, or 7 since they will have a much longer support period. (Albert with a higher upfront cost)