The answer may differ on what is your final goal.
a) why do you need to use 4 sql commands? One command should be OK to get all information. In example:
SELECT * FROM table_name;
will report all data at once within a SqlCeDataReader.ExecuteReader() call that you can iterate to fill a list.
b) if no SQL server will be invoked later (for remote access/sync etc) and if not too much data records, you may consider switching to another data storage (ie xml (slow too) or binary file).
please provide more details if you need more help.
There are also SQLCE examples available here in stackoverflow: Local database, I need some examples and others (use search).
OK, from your comments I see you have some issues getting started?!
At http://www.codeproject.com/Articles/310378/A-Restaurant-and-Waiter-helper-app-in-WPF-and-Wind you will find a complete POS solution. You can change the waiter's code to use a local database.
...maybe adding some simple example later...