Whow,
What a words flowing.
What is needed:
50 times (Hz) in a second (20ms) you need a timestamp with the corresponding RPM value.
If this timestamp differs 1 sec on 24 hours this can be wiped away as nilhil for the 4 second measurement you will do.
A dyno run should last depending on the interia weight 4 to 5 seconds.
This means 200 to 250 datapoints.
Paco