When the DDJ-FLX4 is connected to a PC/Mac and the [BEAT FX CH SELECT switch] is set to [1], the far-left button [FX2-1] on Beat FX unit 2 can't be turned on.

Also, when the [BEAT FX CH SELECT switch] is set to [2], the far-left button [FX1-1] on Beat FX unit 1 can't be turned on.