Why do cinema cameras cost too much? Why do professional digital cinema cameras like ARRI ALEXA, RED Epic cost that much? They record 4k so do other cheap cameras?
Look at it from a data rate point of view: a GH4 records 4K at 100 Mbps.  A RED Carbon Fiber WEAPON records at 300MBps.  That's 24x the data rate.  Look at how very much more expensive it is to purchase a CPU that's 10% or 30% faster than "normal" and you might then have some understanding of how much more expensive it is to offer 10x or 30x the performance in a camera body.