It will be at least four years behind bars for the getaway car driver in a deadly Montgomery Township home invasion and robbery.   18-year-old Sybil White was sentenced Friday for her role in the murder of Gwynmont Drive resident Robert Chae almost 22 months ago.   Prosecutors concede that White was the least culpable of the six Philadelphia residents that were convicted of the crime so consequently she received the lightest sentence.   That brought a warning from the judge.   He told her she would be put away for a long time if there were any violations of the 20-year probation period following her time in prison.