Here is how you can set a single centered x- and y-label on a 2x2 plot.
h1=subplot(2,2,1);
h2=subplot(2,2,2);
h3=subplot(2,2,3);
h4=subplot(2,2,4);
p1=get(h1,'position');
p2=get(h2,'position');
p3=get(h3,'position');
p4=get(h4,'position');
height=p1(2)+p1(4)-p4(2);
width=p4(1)+p4(3)-p3(1);
h5=axes('position',[p3(1) p3(2) width height],'visible','off');
h5.XLabel.Visible='on'
h5.YLabel.Visible='on'
axes(h5)
ylabel('test')
xlabel('test')
...and as a bonus, here is how you can set the same xlabel on all plots, without a for loop
h1=subplot(1,2,1)
h2=subplot(1,2,2)
h=findobj(gcf,'type','axes')
set([h.XLabel],'string','XLabel')
