Is the word four a common noun?

No, the word "four" is not a common noun. It is a numeral, specifically a cardinal number.

Here's why:

* Common nouns are general names for people, places, things, or ideas (e.g., dog, city, book, happiness).

* Numerals represent numbers or quantities.

"Four" indicates a specific quantity and doesn't name a person, place, thing, or idea.