Ah, I understand now, I though you had it all wired up using this article http://www.arduino.cc/playground/Main/MAX72XXHardware
but it turns out it's not.
The matrix is wired slightly different in each article so the code I suggested is no use to you. Also the pjrc version has the matrix rotated 90 degrees so this accounts for the letters scrolling the wrong way. I'm not sure why the letters would be backwards but as Crossroads points out this can easily be fixed by reversing the retrieval order of the matrix data. Now the only thing I can suggest for each matrix showing the same letter is you have them cascaded backwards. Looking from the front of your 4x matrix blocks the arduino should be connected to the right hand device (DIN, CLK & LOAD) and that MAX7219 DOUT should be connected to DIN on the MAX7219 to it's left and so on.
EDIT: Just noticed this in the pjrc article.
Beware, the Matrix library wants Teensy to connect to the last chip, and the DOUT-to-DIN chain connects from right to left. That seems backwards, but if you wire Teensy to DIN on the left-most chip as might seem intuitive, your display will end up with groups of 8 instead of one nice, continuous X-axis coordinates.