Use the set command and the position property. Here is one example with zero spacing between the 2x2 axes and a 0.1 frame around them:
figure, subplot(2,2,1),plot(rand(3)),set(gca,'xtick',[],'ytick',[])
subplot(2,2,2),plot(rand(3)),set(gca,'xtick',[],'ytick',[])
subplot(2,2,3),plot(rand(3)),set(gca,'xtick',[],'ytick',[])
subplot(2,2,4),plot(rand(3)),set(gca,'xtick',[],'ytick',[])
ha=get(gcf,'children');
set(ha(1),'position',[.5 .1 .4 .4])
set(ha(2),'position',[.1 .1 .4 .4])
set(ha(3),'position',[.5 .5 .4 .4])
set(ha(4),'position',[.1 .5 .4 .4])
Instead of getting the axes handles as figure children, you could also save them during creation as subplot.
