when doing commercial wiring can you put mc cable in a 4x4 extension ring

Sure you can.

[QUOTE=stanfortyman;2180073]Sure you can.[/QUOTE Thank you
Is it in the code book anywhere

To be honest I don't think so. I would have to look for any reference.
There would not be anything specifically related to MC and x-rings. It would be a general allowance or disallowance.

Here is the best I can find:

314.22 Surface Extensions.
Surface extensions shall be made by mounting and mechanically securing an extension ring over the box. Equipment grounding shall be in accordance with Part VI of Article 250.

Like I said, it is a general allowance.

Better question here is why would you want? To is all the ko's in the box its self being used?