Broad, who described his tackle as "sh--house", fronted the AFL Tribunal on Tuesday evening and pleaded guilty to his rough conduct charge.
Richmond sought a three-game ban, but the tribunal rejected their argument that Broad's clean record - he's never been suspended in the AFL - was a compelling reason to go down from four.
AFL lawyer Nick Pane said a four-game ban was appropriate, given the tackle had potential to cause even more serious injury than it did.
Parnell was concussed but the tribunal heard he isn't likely to miss more than one game.
"There was injury … but there was still a potential for further injury such as a neck injury by the action of a slinging tackle," he said.
Broad's lawyer Michael Tovey said Broad had immediately taken responsibility for the injury, and reached out to Parnell via former Tiger Shane Edwards, who's now in Adelaide's football department.
Broad's text to Parnell included: "I don't want you to accept my apology, but I shouldn't have done it. Sorry mate. It was sh–house."
Richmond argued for a three-game ban given the Tiger's "exemplary playing history", having never been suspended in his 109-game AFL career or in state-league football.
Tovey said he should be able to "cash in" that good record, adding Broad was trying to spin Parnell and take him over the boundary line.
Tribunal chair Jeff Gleeson accepted Broad was genuinely remorseful, but said his actions were "unquestionably dangerous".
"Players must do all they reasonably can not to cause avoidable head injuries … every player is on notice," he said.
Former North Melbourne champion David King might not be happy with the outcome, having described Broad's tackle as a "horrific incident" and a "car crash" and called for at least a five-game ban.
"We've got one chance to stamp this out, and unfortunately, Nathan Broad and the Richmond footy club have to pay a price," King had told Fox Footy.
"If you give that six weeks, it will not happen again for the year … don't tiptoe through this one."
Later on Tuesday, Gold Coast defender Charlie Ballard was to contest his one-match ban for striking in an attempt to face Geelong on Sunday.
He got the ban for a stray elbow that connected with Essendon's Matt Guelfi in Sunday's loss to Essendon.
The third-quarter incident was assessed as striking and graded as intentional conduct, low impact and high contact.
"He's a really valuable player for us in our backline, really strengthens us back there and we'd love to see him get off," Suns co-captain Touk Miller said.
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