I've taken a good look at the CX4 and I see Sierra Papa has metal trigger parts, and a full metal trigger chassis. They can also mill and flat top the upper with a full length picatinny rail, since I've seen the sights and they leave a bit to be desired. All of that doubles the cost of the rifle. You're now in premium AR prices, and I'm much more familiar with AR's. Still a cool carbine.
I've looked up some of the other 9mm carbine suggestions from BB but it seems they are either hit or miss in quality, or expensive, or hard to find parts for, etc. The Ruger PC9 looks fun (and hard to find), even if it's not "tactical" looking, and Ruger is still in business. In the end I might just go the AR route with metalform mags.