Great stuff.
For example a 57.0 400 with a cadence of 211 total strides is an average of 3.7 strides per second. This depends on how long your stride is as well.
There is a distinction though in the length of your stride and the turnover rate. The stride length in faster stronger runners with more force off the ground is a bit longer and there have been many studies comparing stride rate to length.
In a 400m you do want to increase your cadence as you were saying- without unnaturally shortening your stride though.
If you get around the track in 202 strides in 57 you may be able to run 56.5 with the perhaps 203 strides. Per minute cadence for these times would be 207 in the case of the 57 and 214.5 in the case of the 56.5. The most steps are taken in the first 100 with the shorter strides. The last 100m as one is slowing down your cadence will also slow.
203 strides would be an average of about 6'5.5", at some point and probably on the back straight that stride length is at least 2m or about 1.5" longer.
Agree with what you are saying, but just want to point out you can also lengthen your stride along with a higher cadence. That is the real goal, at least for me.
240 per minute is extremely fast and many runners at this age will never be able to achieve that. Charles Allie is unreal. 4 strides a second!
Great article here on both stride frequency and stride length in a 400:
https://zigapskraba.com/2016/09/01/stride-length-vs-stride-frequency-in-the-400-metres/Van Nierkerk went from 3.98 strides per second to 3.50 over each 100m section. Michael Johnson had a much higher stride rate- 4.31 to 4.11 strides per second. 160 strides compared to MJ's 180.5 strides.
Even though Niekerk is shorter he had a longer stride.