Rumours that the actress may leave her popular role as the Dowager Countess at the end of the third series of the show emerged earlier this year, with producers at first denying claims.
In an interview with TVLine at the Television Critics Association's summer press tour, showrunner Fellowes revealed more information on Smith's future with the series.
He said: "I can't tell you, 'Oh, yes, she's definitely contracted [for series four]' - because she isn't. It's entirely up to her when she stays and when she goes."
Smith opted out of signing a long-term contract for the show in March, while stars Elizabeth McGovern, Hugh Bonneville, Michelle Dockery and Brendan Coyle all made deals to return for potential fourth and fifth series of the show.
"With Maggie, you never know," Fellowes added. "Other [actors] sign on for the next series and they sign for this and they sign for that, but she doesn't. She stays her own person.
"But as far as I'm concerned, hope she stays with the show until Downton falls down."
Shirley MacLaine is the newest addition to the series, playing the American mother of Cora in the upcoming third season.
Star Joanne Froggatt has described MacLaine and Smith's scenes together as "magical."
Watch the cast of Downton Abbey discuss series three below: