To use a Raspberry Pi 4 in combination with a Motorola Lapdock one has to customize the screen resolution to 1366 x 768 pixel.
To set the screen resolution insert in:
$ sudo nano /boot/config.txt
the following lines:
# Motorola Lapdock setting: hdmi_group=2 hdmi_cvt=1366 768 60 hdmi_mode=87
Save with ctr+o, return and ctr+x.