A coupe is always lighter than its corresponding convertible, has greater torsional rigidity and therefore a better handling. It is no accident that all sports cars are coupes and not convertibles.
http://youwheel.com/home/2016/06/20/...ehensive-list/
The reason the Z4 is only convertible is probably related to the poor sales of the old Z3/4 coupe, people see the Z4 only as a fancy convertible and not as a real sports car.