As already has been suggested, I'd use the Gradebook setup to get this to work. Here are the steps to follow to make this work.
In site administration > Grades > Grade category settings make sure to have the following aggregation methods enabled:
- Mean of grades
- Weighted mean of grades
- Simple weighted mean of grades
In addition to the natural aggregation that was already discussed.
Then go to your course, open the Course settings menu and go to Gradebook setup. At the top of the gradebook setup table, there will be a folder icon and your course name. To the right of this, you'll find an Edit link with a menu. Edit the settings at this level to set up the proper aggregation method. I'd select Weighted mean of grades.
Then use the Add category button to add a grade category sub folder, to move the 14 topic quizzes to. Set the weight of this sub folder to 4,0.
Next, set the weight of the global quiz to 6,0.
This will result in the following:
The average grade of the 14 topic quizzes will be counted 4 times, the grade of the global quiz will be counted 6 times.
4 * av grade Topic Quizzes + 6 * grade Global quiz / 10 = course grade
All in all, setting up the gradebook is quite a challenge, but I hope this will help you. In any case, refer to https://docs.moodle.org/34/en/Grader_report for all information on this very interesting topic.