In order to use 4D Engine, you need to have purchased the 4D Engine product. Then you'll have a separate license card for 4D Engine. You cannot substitute your regular 4th Dimension license number for the 4D Engine license number.
If you don't have the 4D Engine license number installed, you'll get an error when you compile and merge your database. If you try to run the resulting merged application, it quits immediately on startup.
If you have a 4D Engine serial number, you can enter it from a regular copy of 4D, 4D Runtime, 4D Runtime Classic, or 4D Server. Open any database, it doesn't matter which one. Go to the Design Environment, open the Database Properties dialog, then click the "License" button in the lower-left corner of this dialog. This displays the license entry dialog. Alternately, you can press Ctrl-Shift-F8 (Command-Shift-F8 on Macintosh) to display the license entry dialog.
Once you've entered the 4D Engine license number, you'll see it listed in the "License Number Entry" dialog. If you expand the directory listing for 4D Engine to show the individual items licensed under this serial number, it will show an item for "4D Engine," and to the right of it the quantity of licenses will be "unlimited." This is how it should look after the 4D Engine license is properly installed.