Jusr started watching - Michael states in modules 7 that 4 is the maximun number of columns in the metadata search.
Not so, click on the drop down and you can add columns.
I went and tried this out to see if I could add columns and to my surprise you can.
Added two additional columns and as far as I could tell it worked just fine.
Also when back and reviewed video 7 (Library Filter, Collections & Smart Collections) and they do state 4 columns is the max.
Maybe it does not function correctly with more then 4 columns, but as far as I could tell it worked just fine.