I found the formula here to calculate the db_work_mem.
By default it is set to 100 connections.
Total RAM * 0.25 / max_connections
I have 4096MB * 0.25 / 100 = ~10MB
In the postgres.template.yml template the db_work_mem: "10MB" is default so it calculate with this formula I think. I think this 10MB is the max right now. Thanks Jay 