Get the 45 version, you'll be glad you did.
Does anyone make a 20 or 30 round mag for that?
Sierra Papa is a small company that makes upgrade parts for the CX4. Their website is: http://sierrapapacx4.com/. I just checked the site and the pics aren't coming up for some reason.
Brownells shows a replacement guide rod assembly, but it's shown as "out of stock/discontinued".
JoeR