You're mixing the plot number up with the element of the series, try:
% set the discrete values for plotting (n in x[n])
n = 0:10;
% set the collection of z's for plots
z = [-2,-.5,.5,2];
% iterate over the subplots
for plot_n = 1:4
% calculate x[n] for current z
x = z(plot_n).^n;
subplot(2,2,plot_n)
% plot x[n] vs n
stem(n,x)
% title it with current z
title(z(plot_n))
end
