He told TV Guide that the TNT drama's team of con artists will evolve in surprising ways in future episodes.
"The fun is still there [and] the heists are still there," he insisted. "But ... [the team's] relationships are going to develop in ways that are profound."
Devlin also promised that viewers will see a new "intensity" in the relationship between expert thief Parker (Beth Riesgraf) and computer hacker Alec Hardison (Aldis Hodge).
"The Parker-Hardison relationship really goes to a much higher level this year," he said. "[There's a] much deeper intensity."
Former Mad Men star Michael Gladis was previously reported to be appearing in the new season's third episode as PR expert Reed Rockwall, while Lethal Weapon's Danny Glover will play a retired war hero in the fifth episode.
The fourth season of Leverage is scheduled to premiere on June 26 on TNT.
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