Before you pick a language, work out where the four boxes are going. Ingest and write to Postgres is an I/O shaped workload, and if your Python is spending its time waiting on the network and the database, a rewrite in anything compiled buys you far less than you expect. Profile it for an afternoon first.
In my case the honest answer turned out to be that we were spending most of our CPU in JSON deserialisation and in building one INSERT per row. Swapping the JSON library and moving to batched writes cut our box count enough that the rewrite got deferred by a year. If yours is genuinely CPU bound in parsing then Go will help a lot and Rust would help slightly more, but find out which world you are in before you spend six weeks.