[OS X TeX] ifthenelse and an empty value

[Ueliisa]

I try to make an \newcommand that could have 3 or 4 parameters.
With \ifthenelse I want check about is the 4. value set or not an then 
use another command set:

\newcommand{\newcmd}[4]{\ifthenelse{#4={}}{#1 - #2 - #3}{#1 - #2 - #3 - 
#4}

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