I think, following may be useful when GUI scripting. It will act on front editor window. When executing as is from Script Editor (or Script Debugger) it will act on script content:
--- script: Move the caret several positions
-- written: by KniazidisR, today
tell application "System Events"
key down 63 -- holding Fn key
-- move carette 4 times down
repeat 4 times
key code 125
end repeat
key up 63 -- release Fn key
end tell
-- NOTE:
-- to move up, use 126
-- to move down, use 125
-- to move right, use 124
-- to move left, use 123