You can't see the fourth plot because the plots for 'c' and 'd' are overlapping. You can run the "fplot" commands one by one and observe the plot for 'd' overlapping the plot for 'c'.
You can also use the "legend" commands to show legends in your plot and see the which color represents which plot.
You can run the following commands:
>> fplot(a);
>> axis([0 500 0 0.035]);
>> hold on
>> fplot(b);
>> fplot(c);
% Observe the figure before running these
>> fplot(d);
>> legend('a','b','c','d')