method seekRelative

This method changes the position of the attached stream relative to its current position by the amount specified in offset. A negative number can be used to move backwards from the current position.

var fileStreamObj = shell.serviceManager.basics.fileStream;
var streamReaderObj = shell.serviceManager.basics.rawStreamReader;

fileStreamObj.openForRead("c:\\temp\\test.txt");
streamReaderObj.stream = fileStreamObj;
streamReaderObj.seekRelative(4);

fileStreamObj.close();