Fourth Normal Form (4NF)

Fourth Normal Form comes into picture when Multi-valued Dependency occurs in any relationship.
Rules for 4th Normal Form

For a table to satisfy the Fourth Normal Form, it should satisfy the following two conditions:

A table is said to have a multi-valued dependency if the following conditions are true,
For a dependency A → B, if for a single value of A, multiple value of B exists, then the table may have a multi-valued dependency.
Also, a table should have at least 3 columns for it to have a multi-valued dependency.
And, for a relation R(A, B, C), if there is a multi-valued dependency between, A and B, then B and C should be independent of each other.
If all these conditions are true for any relation(table), it is said to have multi-valued dependency.

As you can see in the table above, student with s_id 1 has opted for two courses, Science and Maths.
It has two hobbies, Cricket and Hockey.
You must be thinking about what problem this can lead to, right?
Well, the two records for a student with s_id 1, will give rise to two more records, as shown below, because for one student, two hobbies exist, hence along with both the courses, these hobbies should be specified.

And, in the table above, there is no relationship between the course and hobby of the column.
They are independent of each other.
So there is multi-value dependency, which leads to unnecessary repetition of data and other anomalies as well.

To make the above relation satisfy the 4th normal form, we can decompose the table into 2 tables.

Now, this relation satisfies the fourth normal form.
A table can also have functional dependency along with multi-valued dependency.
In that case, the functionally dependent columns are moved in a separate table and the multi-valued dependent columns are moved to separate tables.
If you design your database carefully, you can easily avoid these issues.