Since this form was developed in 1970; and as it only applies to many to many relationships; which are not allowed in modern relational databases – unlike in hierarchical databases from 20 years ago, when M:N was common – this form is nowadays ‘technically’ redundant.

In practice the only benefit of applying this rule is for very large data schemas; it removes the inevitable duplicate columns in data tables, and ensures that all of the table structures created are only as large as they must be. It is good practice to apply this rule, if the database under development is very large e.g. millions of records.

So to summarise Normal Forms - the first three Normal Forms are really all you need to know. As you will by now have realised these first three are complicated enough and they are sufficient for any general business database application.
To be able to Normalise data effectively, then you need to practice as this is the only way to learn properly.

To consolidate your understanding of the Normalisation process; it is recommended that you have a go at the interactive Database and Normalisation exercises. Have fun with the Exercises and help yourself learn Normalisation by doing them all.