...If you're doing a single stream encode or using software that loads the entire dataset into memory, then yes, most hard drives will no impact. If you're running an encode that's multi-pass or using encoding software that reads the original data forwards & backwards to compress it, then you'll experience disk thrashing...which should be enough said...
The OP was asking specifically about
4k drone video, Premiere Pro, and how SSD vs 2TB Fusion Drive might effect editing or rendering performance. He was not asking about performance using 3rd party transcoding tools, nor was he asking about using a USB 2.0 external drive. Nobody in their right mind would edit or export/import
4k video from a USB 2.0 drive.
I just did several multi-pass encoding tests using H264 4k video from a DJI Phantom 4 on both Premiere and FCPX, using both a portable USB 3 bus-powered hard drive and the local SSD drive of my 2015 iMac 27.
Results:
(1)
No difference in Premiere multi-pass H264 encoding/export performance whether media was on a 100 MB/sec portable USB 3 drive or an 1800 MB/sec SSD.
(2) Total data volume and I/O requests read/written for multi-pass encoding of an H264 4k file is very low, averaging about 1.4 MB/sec reads and 2.2 MB/sec writes over the encoding period. This is well within the performance capability of a USB 3.0 external drive or a Fusion Drive.
(3) Premiere CC 2017 H264 export is highly CPU-bound, with all cores at 100%. This alone tells us it's not I/O-limited in the slightest, otherwise it would be waiting on I/O, and CPU would be lower. However Activity Monitor gives the actual I/O performance numbers which confirms this.
I'm not particularly advocating he get the 2TB Fusion Drive iMac, only that it won't make much performance vs SSD difference for a few small 4k drone videos. However if he does much video work he will rapidly need an external drive, so in that case why not get the SSD iMac? He will also need a backup drive so he'll actually need two external drives.
Even though pure exporting to H264 does not require a fast drive, other editing tasks may. This includes editing multicam 4k from lower-compression codecs such as ProRes or DNxHD. So whatever external drive he gets, it should not be slow.
Although he was interested in Premiere I tested the same clip in FCPX and the export numbers are below:
Material: 1 min 30 sec 4k H264 video from DJI Phantom 4 drone, size=681 MB, bit rate = 60 mbps
Premiere CC 2017 multi-pass export to 30 mbps 4k H264: 7 min 19 sec
Premiere CC 2017 single-pass export to 30 mbps 4k H264: 3 min 41 sec
Premiere CC 2017 export to DNxHD MXF: 3 min 1 sec
FCPX 10.3.2 multi-pass export to 30 mbps 4k H264: 2 min 11 sec
FCPX 10.3.2 single-pass export to 30 mbps 4k H264: 1 min 7 sec
FCPX 10.3.2 export to ProRes 422: 1 min 3 sec