Let's take the definition from Wikipedia:
For real data you have then the symmetry:
In your case [itex]N=4[/itex], and you have [itex]X_0=3,X_1,=1,X_2=-1,X_3=1,[/itex] all real.
From the symmetry property you should have [itex]X_4=X_0,X_3=X_1,X_2=X_2[/itex].
And you have it ([itex]X_4[/itex] can be thought of as defined by the symmetry property).