I assume that your new iMac has only a FW800 socket so your FW400 plug won't work.
You can get a cable with the camera's 4 pin socket on one end and the new 9 pin plug on the other.
(The old FW400 had a 6 pin plug).
Best place to look for a cheap one is Amazon.
This is what you need.
Message was edited by: Ian R. Brown