Mitch,
You are correct, the 4000M does not support sFlow. Unfortunately, from
what I can see the 2520 does not support sFlow either. However, if you
enable sFlow on the devices that you can, you should see a reasonable
view of the overall network traffic, since any up- or down-stream
switches which do have sFlow would also see the traffic.
The 5400 and 4200 switches both support sFlow, if you are thinking or
replacing the 4000s with these, or any other sFlow switch, then this
would of course increase your overall network visibility.
Stuart