Unlike Rekordbox, which is "plug-and-play" out of the box with a license included, using the DDJ-FLX4 with VirtualDJ requires a specific setup process.
If everything works, the FLX4 will light up beautifully. The pads will default to Hot Cues, and the Beat FX section will respond to VDJ's effects engine. pioneer ddj flx4 virtual dj