The Pilgrims thrashed a revived Newport County side 6-1 at Home Park on Saturday to ensure that they can no longer be caught by Luton Town in fourth.
It was a slightly more stressful afternoon for Portsmouth, meanwhile, as Jamal Lowe scored twice in the final 13 minutes to overcome Notts County 3-1.
All three automatic promotion places have now been wrapped up in the fourth tier, with Doncaster Rovers getting over the line last weekend.
At the other end of the division, Leyton Orient recovered from a goal down against Hartlepool United to win 2-1, meaning that they can still mathematically avoid the drop into non-league football in the remaining three weeks.
Newport remain in the dropzone after their heavy loss on the south coast, one point adrift of Hartlepool in a tight race for survival.