Your Macbook Pro has some decent specs; but data transfer for 4K production will be painfully slow from the internal HDD. You're going to need a fast Thunderbolt RAID to serve up those huge 4K files, even if you upgrade the computer … so I suggest that you invest in a Thunderbolt RAID before buying a new iMac.
Plus, inexpensive to upgrade the RAM in your MacBook Pro to 16GB. That might be all you need for now.
Then I'd save my cash, wait for the second generation of the 27" iMac with Retina Display (2016?). Buy it with an SSD, upgrade the processor. Bring it home, plug in my Thunderbolt RAID, good to go!
