In short: don't go faster than 1/200
If your problem is a black band on your images (or overall darker images), you are reaching the sync speed of the 5D mark IV (see What is sync speed?). As 1/200 is the maximum sync speed of the Canon 5D IV (info from Canon), your frame won't be exposed correctly if you try going faster than 1/200. In short, the curtains of your body won't get fast enough.
Example from http://neilvn.com/tangents/high-speed-flash-sync:
HSS (High Speed Sync) doesn't get rid of the problem but make the flash works in a different way, as you can see in this comparison (from http://neilvn.com/tangents/maximum-flash-sync-speed)
Now a short explanation (taken from http://neilvn.com/tangents/maximum-flash-sync-speed) of what exactly does HSS :
So with the older flash technology, flash is dissipated as that high-energy burst of light .. but camera manufacturers came up with
the stunning adaptation of that technology, where they dissipate the
energy from the flash as rapidly pulsed light. In effect, the flash
now becomes continuous light over a very short period. The light from
the flash is now dissipated even as the shutter curtains move across
the frame. As that window between the two curtains move across the
frame, the light from the camera’s speedlight is dissipated … exposing
correctly for the entire frame. Remarkable technology!
But .. and yes, there is always a but … this comes at a price.
Instead of the energy from the flash being dissipated now as a
high-energy burst, the light from the flash is now dissipated over a
longer period. This means the effective power from our speedlights is
reduced when we switch to high-speed sync mode, instead of the
old-school way of triggering our flash as that high-energy burst of