Cyclic prefix length in baseband OFDM with ZF equalizer
The channel which you have created is having 4-Taps and all taps are one after the other, meaning roughly there are 4 multi-paths and they are very close to each other. How close depends on what is the Sub-Carrier Spacing you would have assumed. Anyway, the point is only 3 samples of Cyclic Prefix would be enough and even with 0 or 2 samples of CP will not cause any noticeable degradation. The degradation is mostly due to AWGN which you are adding. 

If you want to see the impact of 4-Tap channel, then I will suggest you to have larger delays multiplied to the magnitude of the four taps. One way to do it is as follows:

h = [.7*exp(1i*pi/4) zeros(1,50) .4*exp(1i*pi/3) zeros(1,75) .2*exp(1i*pi/6) zeros(1,100) .1*exp(1i*pi/5)]; % 4-taps channel


This will create four paths channel with 2nd, 3rd and 4rth path coming at 50, 126 and 227 samples respectively and hence the 2nd, 3rd and 4rth copy of one OFDM symbol will interfere with next OFDM symbol more.

With your current h assignment, all you will have is intra-symbol interference which can be delt with no CP since the channel is just 4-taps with no delays in between.

Also, consider removing AWGN and varying the amount of zeros I have added in between 4-Taps of h, to actually notice the impact of multipath inter-symbol interference and advantage of 512 length CP over 0/2 length CP.