The pair hit the rocks last season after they adopted a baby, Robert had a heart attack and Kitty almost had an affair.
Season 3 ended with the two separating but Lowe says things will change in the new season.
"Last season was the study of a disintegration of a marriage and we begin this year worlds apart," he said.
Lowe says a tragedy will strike early on that ultimately help bring the two together.
"It's going to be what they needed to try to put the pieces back together ... and pretty romantic," he said.
The fourth season of "Brothers & Sisters" premieres Sunday, September 27th at 10 p.m. EDT, 9 p.m. CDT (0100 GMT Monday).
On the Net: