Maggie here, a Shopify Guru. :)
It's not possible to add more then 3 variant options on your products. You can have up to 100 variants per product (that is 100 unique combinations) but only three options or "categories", if you will (ex. color, size, shape). This is true of any theme and how our platform is natively set up. You can read more about that in this doc about variant limits.
That being said, there are several different apps that we have in our app store that allow you to add "options" to your products in different ways.
Check out some of these! Infinite Options is free. :)
Hope this helps! You can contact support 24/7 for any further questions!
You can add the 4th option as a custom property or use metafield to add additional characteristics to your product variations
hope this helps