if you have 4 cameras, 1 tm751, and motion sensors for each camera:
if you only have one tm751, you have to have all of your items on the same house code. The tm751 is what will send and receive all of your on/off codes for all your cameras and motion sensors.
you will probably have to do the simple mod of covering the dusk/dawn sensor on the motion sensor to get this to work. Use the link that Puck gave you above.
evidently you have the motion sensor located in a place where it gets sun sometimes and then shade. Which means what is happening: when you have motion from motion sensor (c1) it transmits c1 to the powerline through you tm751, your tm751 puts c1 on the powerline, which turns cam1 (also c1) on.
what is happening is that the motion sensor gets a little shaded and sends the (c2 on the dusk/dawn code, which is also cam2). When the tm751 sends c2 on, cam1 (c1) goes off and (c2) goes on. Unless you disable the dusk/dawn sensor by putting black tape over it this setup will always cause you problems.
Do the simple mod and I'll bet your problems go away!