Matlab Arduino Hand Shake

Hello everyone, I'm trying to get my arduino up and running so that I can use matlab to control it. I followed the instructions in the following video,

https://www.youtube.com/watch?v=ymWXCPenNM4

I mistakenly entered the wrong port, and as expected an error appeared stating that I had to pick from com1 or com4, the arduino is set up on com4 so I changed the variable and ran the script again. This time I get an error that only com1 was available. I typed instrfind into the command window and found that com port 4 is closed. How do I go about turning it on? Thanks Zack

   function [S, flag] = ArduinoHandShake(comPort)
  % Initialize the serial port communication between matlab and Arduino
  flag=1;

s=serial(comPort);
set(s, 'DataBits',8);
set(s, 'StopBits',1);
set(s, 'BaudRate', 9600);
set(s, 'Parity', 'none');
fopen(s);
a='b';

while (a~='a')
    a=fread(s,1,'uchar');
end
if (a=='a')
    disp('serial read');
end
fprintf(s,'%c','a');
mbox=msgbox('serial communication setp.'); uiwait(mbox);
fscanf(s,'%u');
end