Focus Screen Question

Would a Ec-CIV focusing screen from a 1DMkIII work on a 1dMkIIn that normally uses a Ec-CIII screen without any issues? I know that there is a custom function setting within the 1D that you are able to choose your focusing screen in but this is where my confusion comes in.

On the 1DMkIIn menu it does not have an option for the CIV.
On the 1DMkIII menu it has an option for the CIV and it is a separate option from the CIII screen.

What sort of issues would I experience if I used the CIV in the 2n?

The focusing screens affect light metering. The settings tell the camera to compensate for that. If you use a screen without the proper setting, either due to forgetting or because it's not supported, it may be that your exposure will be off by some amount, in one or the other direction.
If you use an external meter, this is no issue. Or you have to learn and always set a certain exposure compensation. Depends upon how much the exposure is off.

Hmm. I guess the real question is is what the differences between the CIII and CIV are. If no one has personal experiences with the two then I guess I need to do some testing.