Kind of Graphic Error in Rekordbox 4.0

@Leon > If you are on a PC, please try updating your graphics driver.

Depending on your chipset, it may be one of these:

http://support.amd.com/en-us/download
http://www.intel.com/p/en_US/support/detect

You can also try lowering the graphics setting by going to Preferences > View and change the Waveform option to a lower value.