I am generally reluctant to use a 4-core machine, because my work often involves getting work done while other stuff happens, like clone backups—and cloning takes a lot of CPU power—see the rsync processes below. On a 4-core iMac 5K, about 60% of the computing power is being used for these in-progress clones and while there are several at once, the I/O speeds are relatively low, which reduces the CPU load.
Bottom line: a 6-core or more machine is much better for workloads where background tasks are chewing up CPU time. It is one reason among several that I might end up with an iMac Pro instead of the 2017 iMac 5K. The only mitigating factor with the about $3699 top-end 2017 iMac 5K is that its 4.2 GHz CPU is the fastest available, matching or even outrunning my 8-core 3.3 GHz 2013 Mac Pro on some tasks.
$1599 SAVE $150 = 8.0% NEC PA302W-BK-SV 30" 16:10 IPS Monitor with SpectraView II in Computers: Displays