Monitors often have different operating modes in which they have different response times. Often there is a mode that completely kicks the quality to the curb and only focuses on the response time, with a correspondingly large amount of overshoot etc. - the purpose of this mode is merely to achieve the 1ms response times so that the manufacturers can claim them, because nowadays every monitor manufacturer specifies a 1ms response time. Realistically, however, the stated 4ms is more likely for non-OLED displays if you also want to see something on the screen (without overshoot and smearing and so on). 4ms is also a pretty good result as far as that is concerned.

So the 4ms in the text will tend to refer to qualitatively usable operating modes.

I recommend watching the review video of the Gigabyte M27Q from Hardware Unboxed. There, the monitor is analysed in detail and everything is explained in detail.