Yeah! That's what i think, but then, the transfer speed in USB3 was enough for editing 4K files...
In general the main challenge with 4K editing is not I/O but CPU. Whether your I/O is SSD vs HDD RAID makes very little difference. The second challenge is GPU, not for editing per se but for effects. However I/O can become a factor if you must transcode the 4K to get good editing performance, but then that often won't fit on any SSD you can afford.
4K is so CPU intensive to edit that you generally must transcode to a lower compression codec for smoothest performance, and definitely when editing multicam. This is even using FCPX which performs better than Premiere CC and even on a top-spec 2015 iMac 27.
When you transcode to proxy -- not optimized ProRes, just proxy -- this roughly doubles the media size. Add in scratch files, cache files, render files, etc, and your space requirement can easily be 5x the initial out-of-camera H264 media size.
This means few people could afford SSD storage of sufficient size to hold all that -- it's just too big at transcoded 4k resolution. Also you don't need the extra SSD performance. About 500 megabytes/sec would be great and that's easily attainable by a 4-drive RAID-5 array. If you pay 8x as much and get an SSD array which supports >= 1,000 megabytes/sec, it will make very little difference in the overall editing performance.
I have six Macs, three are Fusion Drive and three are SSD. I have done lots of side-by-side testing with my 2013 iMac 27 with 3TB Fusion vs my 2015 iMac 27 with 1TB SSD. There just isn't that much difference in boot time, app loading time, etc. SSD can be a little better and is a little more consistent but it's typically not a huge difference in real world performance. Your media must often be on an external drive anyway, so in that case you may as well have an SSD boot drive even though it doesn't help real-world editing performance much.
A fast 7200 rpm 3.5-inch USB drive is fine for editing a single stream of H264 1080p video. I would not recommend editing with media on a 5400 rpm bus-powered drive, and even a 7200 rpm 2.5-inch bus-powered USB drive like the 1TB HGST Touro S is marginal:
https://amzn.com/B00IVFDQ48
The Seagate Backup Plus Fast is one of the fastest USB 3.0 bus-powered drives. It is internally RAID-0 and you could probably edit some 4k from that:
https://amzn.com/B00HXAV0X6
For 4k you ideally need at least a two-drive RAID-0 array, like the G-Tech G-RAID:
https://amzn.com/B00846Z4YY
One of the least expensive of this type is the 6TB USB 3.0 WD Mybook Duo:
https://amzn.com/B00KU686JQ I have tested the 8TB Thunderbolt version vs the USB 3.0 version and they are equally fast. However Thunderbolt is nice and convenient and due to Mac firmware limitations you cannot boot from a USB hard drive over 2TB in size. So if you need to clone it then boot from it, the drive must be Thunderbolt.
Another issue is perceived durability. I have several of the MyBook Duos I use for backup, but they get pretty hot and I'm not sure if I'd trust them for 24x7 production use, month after month. The G-Tech has a cooling fan and metal frame and is probably better suited for continuous production.
The biggest challenge in 4K is CPU not I/O. You will need the fastest machine you can possibly afford, and even a top-spec 2015 iMac 27 can bog down under that load. If you are editing 5 minute long 4k iPhone videos, almost anything will work, including a MacBook Air. If you are editing significant amounts of material, the fastest possible machine is needed, and if you must transcode that in turn requires lots of disk space and higher I/O performance, but not SSD-level performance.
Remember whatever storage solution you get, whether HDD, SSD, internal or external should be backed on on separate physical storage.
