RE: how to add priority box on case status
Hi partner,
You can follow the steps bellow.
1.Open the Form editor of case.

2.Open the property page of the priority field.
3.Go to the Details tab and click Edit.

4.Click + to add your forth option.

5.Save and publish your changes.
Then you can see the new option you just added in the priority field in your case form.

Hope it helps.
Best Regards
Leo