The Smart CFX button is still mapped in version 2.9 of the DDJ-FLX4 Unofficial MIDI Mapping. You can copy the settings from there.
In version 2.9
- Smart CFX button was mapped to toggle FX2/FX3 on decks 1/2 (on/off).
- this let you manually set FX2/FX3 and then toggle them on/off with the Smart CFX button
In version 3.0
- Smart CFX is un-mapped/free