Loose handlebars are an easy fix on the D4+. There is a small screw (2.5mm drive) located in the hinge of the handlebar:
Begin by sliding the collar out, and folding the handlebar into the lowered position. Once the screw is accessible, it is time to adjust it.
The trick behind adjusting this screw is to not completely tighten or loosen the screw, but to instead find the "sweet spot" where it presses firmly against the inside of the handlebar. It is a narrow window, so it is recommended to adjust the screw an 1/8th of a turn at a time. You will know when the handlebar is properly tightened when the handlebar has no play when locked, but can easily be folded down. If you are having troubles keeping the screw in place, it is recommended to add some grease to the screw threads. This will remove any vibrational forces that may loosen the screw, while allowing it to be easily adjusted.