Hi,
I am developing a SCADA system for up to 4000 tags in total to show the trends of and about 400 parameters for recipes in total. My question is which type of database (e.g. Postgre, MSSQL) is the best? SCADA will not exchange any data with any ERP system.
You describe what I would consider a very light load. Any of the major brands, free and not, should work fine. I happen to treat Microsoft products like the plague, but your mileage may vary.
Another option in 8.3 is the built-in Core Historian (QuestDB). I would choose this option only if you are planning to install the database on the same server as Ignition. If not using the Core Historian, plan to use separate resources.
The 'which database' decision often involves many people, and those that will be maintaining the system often carry the largest vote. As in, if 'the company' has a team of (1 or more) DB admins which specialize in MSSQL, then MS is an obvious choice.
As a personal preference, each time I have the option to work with Postgres, I look forward to it.
Core Historian is a waste of time here, he has non historical data to store, so he needs a normal DB
Good point. Though, the addition of a SQLite DB could be an option for recipe management. 
Are there many resources that will help me work with Postgre (manuals, community)?
You can check Inductive University. It has basics on how to configure connection to the DB and videos on typical SQL operations in Ignition.
Disclaimer: I’ve never tried PostGres in my life, but I’ve heard feedback that it’s the nerdy nerds choice of database.
I’m a MariaDB man. Word on the street is, MariaDB is easier to get started with than PostGres. Depends on your background I guess, my degree is in Mechatronics not Comp Sci.