The No. 4 Chevrolet, which will carry sponsorship from G&K Services, was run as the No. 1 in 2013 with veteran Mike Wallace handling the driving duties.
Cassill made 23 starts for the team this past season in a separate entry, finishing 24th in the series' points standings.
Dave Fuge, who also joined the organization this past year, will return to serve as crew chief, according to a team release.
Team owner Johnny Davis said he was pleased with Cassill's progress in '13 with the team and is looking forward to having the combination together for the full season.
"He's a good driver," Davis said, "a smart guy and easy to work with. We think we can do some good things together as we increase our involvement for 2014."
Cassill will also likely compete fulltime in the NASCAR Sprint Cup Series. He made 33 starts this past year with Circle Sport Racing.
"It's big shoes to fill," Cassill said of the JD Motorsports effort. "I feel like Mike Wallace had a big part of making that team what it is and putting it on the map.
"The more races, the better for me. It's going to be fun on the Nationwide side. Dave knows a lot about race cars and Nationwide racing in general, and it's been fun working with him. I think we have a lot of faith in each other."
Officials say they expect to run a partial schedule with a second car -- based on sponsorship opportunities ?- with Canadian driver Daryl Harr.
Cassill had a best finish of 17th last year in the organization's second entry.
Wallace, a four-time winner in the series, had driven for Davis since 2009. He finished 17th in this year's points standings, with one top-10.
"Mike and I go a long way back, and his input with this team has been important," Davis said. "The Nationwide Series is a lot tougher and more competitive than it's ever been, and we had some good races and good seasons together."
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