Step 1:
Pāṇini classified grammatical rules systematically in Sanskrit grammar.
Step 2:
The major categories include:
\[
\text{Vidhi, Niyama, Pariśaṅkhyā, Adhikāra, Atideśa, Apavāda}
\]
Step 3:
The total number of principal types is:
\[
6
\]
Step 4:
Hence, the correct answer is:
\[
\boxed{\text{(4) 6}}
\]