You data is not in correct order. The axis scale is also not proportional. Try geoplot if you have mapping toolbox.
x = [126.509 126.508 126.511 126.512];
y = [37.403 37.4048 37.4081 37.4063]; % The original data order is incorrect
% geoplot
figure
geoplot(y([1:end 1]), x([1:end 1]))
geobasemap streets
% normal plot
figure
plot(x([1:end 1]), y([1:end 1]))
axis equal
