According to Adam Schefter of ESPN, the 49ers also plan to offer Atlanta Falcons offensive coordinator Kyle Shanahan a deal of the same length.
The 49ers have to wait until after Super Bowl LI to officially hire Shanahan, who is now preparing his Falcons to face the New England Patriots. The Falcons left for Houston on Sunday.
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San Francisco's previous two head coach hires, Jim Tomsula and Chip Kelly, both received four-year deals.
Shanahan met with the 49ers for a second time on Friday. He and the 49ers had a lengthy interview with Minnesota Vikings assistant general manager George Paton, who had been assumed to be the frontrunner for the job, on Friday and Saturday. Shanahan had originally interviewed with the 49ers on January 6.
Arizona Cardinals vice president of player personnel Terry McDonough also interviewed with Shanahan and the 49ers on Saturday.
Shanahan has been with Atlanta since 2015 and has nine years of experience as an offensive coordinator. Prior to his two years with Atlanta, he held the same position with the Cleveland Browns, Washington Redskins, and Houston Texans.