Extending circuit from dead-end 3-way switch
Wow.  You actually have /4.  

The answer is it can work, but you may need to flip it.   That is to say, right now, the black wire may be switched-hot going to the lamp, so the wires are


  switched-hot traveler traveler neutral.   


If so, you will need to rearrange other parts of the circuit so the other 3-way is the one with switched-hot and the wires that come here are 


  always-hot traveler traveler neutral.  


Then you can simply tap always-hot and neutral.  

Tell whoever fit the /4, well done.   Normally this question answers with a "nope, unless you like smart switches".