Exactly.
But by "innocent," be sure that this simply means that a lawyer forgot to cross a "T" in some obscure affidavit.
Righto. "Innocent" in this context generally means "improperly convicted," not actually, factually innocent of the crime in question.
More importantly, I would suggest that "innocent" in this should mean "not deserving of the penalty." Some significant percentage of those actually innocent of the crime for which they were convicted had committed others, often many others, they got away with. I believe this is commonly known as "poetic justice."
However, I would like to point out a caveat. A great many threads on FR recently have been about police and prosecutor corruption and malpractice. Many of FR seem to agree that these institutions are out of control.
In which case it would be logical to expect the wrongly convicted number to be considerably higher than 4%.
You can't claim that the cops and DAs are all crooked, and at the same time that (almost) everybody in jail was properly convicted.