Is AWS RDS billing purely based on RAM/IO and storage? or is there any additional per database charges?
For my RDS deploy, If I have 1 PostgreSQL DB that has all my data but only receives 2000 queries per day vs if I have 4 PostgreSQL DBs that have the same relations as the 1 DB but those relations are split up on the 4 DBs and the 4 DBs will collectively receive the same 2000 queries per day... will the bill between the two different setups be essentially the same amount? The assumption being that the "size" of the data in 1DB vs 4DBs is exactly the same.
I want to split the data across multiple databases to make reporting for different modules in my system easier.
AWS RDS charges based on size, Data Transfer, backup, Storage etc. In your case if you are going to keep the size of the instance same then it is better to have only one instance as the costing for instance is more than the Data Transfer and storage. It makes no sense to have 4 same size of instances as the base billing will be 4 times. If you use small instance size then it may make some difference.
I would request you to refer to the below links: https://aws.amazon.com/rds/postgresql/pricing/ https://calculator.aws/#/
With this you can understand how much you are billed for instances based on your usage also instance size. Also you can choose different options to reduce the billing like Reserved instance.
Since there will be only one instance I think the charges will be the same, as long as the parameters on which it charges is the same.