"Anymouse. I mean swing not sway. We swing the left side of the body forward and up(Flight) this swing continues till the end of beat four untill the drift ends."
No, it does not. The UPswing ends at the high point of the step, which is about when the feet pass at the end of step two. At that point, a new swing, a DOWNswing commences.
If we were dancing waltz with foot closure, the boundary between upswing and downswing would be very noticeable as the body flight nearly pauses atthe top. In foxtrot its softer because the body flight continues with only a slowing - but there is still a boundary between the upswing and the downswing.
"My question is :when does the this left side lead end?"
That's not a question about swing (an action), it's a question about a body position.
"Sway is the end result of cbm and swing."
Yes. And side lead is an end result of CBM (or other rotation), with or without swing.
Side lead and sway are both positions.
CBM and Swing are both actions.