stackedplot does not return the axis handles which is needed to place the text objects but you can get the axis handles using the undocumented NodeChildren property. Use flipud to reverse the order of axis handles so they appear in the same order as the stack.
S = stackedplot(rand(50,4));
ax = flipud(findobj(S.NodeChildren, 'Type','Axes'));
text(ax(1),10,.8, 'Text1')
text(ax(2),20,.6, 'Text2')
text(ax(3),30,.4, 'Text3')
text(ax(4),40,.2, 'Text4')
