Before you invest in a new system, try to improve storage speed FIRST by either using SSD drive or raid two standard drives. I edit 4K material (Also 1080 200Mb prores) on imac core i7 3.4Ghz, mid 2011 20Gb Ram. When adding a 500G SSD I got the performance i needed.
An SSD gives about 500Mb/s, 2 Raid drives gives about 3-400Mb/s, both sufficient for 4K editing. External SSD gives the same class of performance if you have USB3.