How to add a 4th Type of Expense since there are only Business, Personal or Split category?

I can see that this is your first post ali11, so let me take the time to welcome you to the Community space.

Currently, the option to add anther category is unavailable in QuickBooks Self-Employed. You can utilize the ones that are generated by the program. These categories are in line with the IRS Schedule C to ensure that your estimated taxes are reported properly. 

At times, we roll out updates based on the customer's request. I'll be taking this as a feedback and share your post with our software engineers for consideration.

For now, you'll want to read this article about the different categories in QuickBooks Self-Employed. This way, you might find the category that you that fits the transaction that you want to record.
 

As we assess this opportunity, I encourage you to visit our blog to stay current on all the QuickBooks News.

Do you have any other questions in mind? You can leave them in the comment section and I'll make sure to get back to you as soon as I can.