Even later than these four historical categories of Vedas (Saṁhitā, Brahmaṇa, Āraṇyaka, Upaniṣad), other texts known as sūtras were sometimes included as well.
However, sūtras are different because they were said to be composed by humans—hence they are “secondary literature” (commentary about the Vedas). Religious texts composed by humans is known in Hinduism as “smṛti.”
All Hindu religious texts are thus either śruti (divine-made) or smṛti (human-made).