Starting in 2018, the kicking team was no longer able to get a running start and in the two years since, only a dozen attempts have been recovered in 115 tries – just 10.4 percent. In that same time span, five of 14 (36 percent) fourth-and-15 attempts were converted.May 25, 2020

Quoted from an nfl.com article.

So prior to that it was supposedly 21% for onside kick recoveries. Not quite equal hence the it should be at least 4th and 20.

It seems odd to me that was only 14 attempts at 4th and 15 so that stat might be only using that exact down and distance. And hence could be a little misleading. I grant that.

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