Problem Can we add another meta table in TimeCard and have four fields rather than three?
Can we add another tag in TimeCard and have four dropdowns in the appointments rather than three?To not make TimeCard too complicated to use, we have maximized the number of tags to three. Maybe you can get the information you need in another way.
The simplest solution for catching the 4th parameter on each appointment is to ask your users to add that information to the subject field. The subject field is also stored in the database, and you should be able to make queries against the database.
You can also consider using Tag Type and Tag Category in a way that gives you the information you need without having to burden the users with selecting from four dropdowns for each appointment.