Better way to full text query across multiple tables?
I do not know how dynamic your queries are, but naturally each table needs a separate query for that table.  

Assuming that you are constantly querying this 4 table group, it might be worthwhile to create an indexed view that included the searchable fields you need from the 4 tables into a single view. 

This means the overhead of maintaining the indexed view, but may help in this case.

Of course, if you have a widely varying array of tables that might be joined, you would need to consider whether this is a good idea for you or whether you should query each table separately as you are doing.