A database can be open in both 4th Dimension and the 4D Compiler at the same time. You don't need to quit 4th Dimension each time you want to compile the database.
To compile a database that is already open in 4th Dimension, you only need to leave the Design environment by switching to the User or Custom Menus environments. Then switch to the Compiler. If you have been using the error file in 4th Dimension, close the file by choosing "Stop browsing error file" from the Use menu in the Design environment.
During compilation, you cannot use your database. Once compilation is completed, you can resume using your database.