Novice here…
My 2 gang junction box had 2 switches, controlling a fan and its light.
I got this fan from HD (Fan) which needed only one hot and a neutral to operate. The remote controls the fan's light. The other switch was wired to 4 new can-lights. The electrician did everything good.
Now, I want to replace the Light switch with a Z-wave dimmer : Dimmer
When I opened the junction box, the existing wiring confused me. The right switch controls lights and left one controls the fan. The new Dimmer needs 4 wires – Line (Hot), Neutral, Ground and Load.
The dimmer switch I have has only 3 
Link to Dimmer switch wiring : Dimmer Switch Wiring
Another view 
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I was able to swap the fan switch with a modern one and it worked fine. However, the Z-wave dimmer I couldn't make it work.
Can someone guide in simple words, how do I wire the dimmer to make it work ?
P.S. The neutral wire hanging down is something I haven't used yet. The dimmer switch said it needed a neutral, so I pulled one from existing lot.
P.S.S. I don't know how the new fan and can lights are wired. I cannot open it either. Sorry!
Thanks!
Best Answer
I can clearly see in the picture provided your neutral ( the one with a pigtail on it ) is not wired to anything.
That's your "4th wire", if I understood your question correctly. Your old switches didn't need a neutral.. Your new ones will... Use "those" neutrals...