‘Ghost’, or ‘ghost mode’ in GranTurismo 4 (GT4) is a semi-transparent copy of your fastest run around that particular track. It’s designed to let you see your fastest pace around the track while you drive. You can even load other peoples ‘ghost save’ to pace (or outpace) them through the course.
I’m no doubt sure it’s useful, but I can’t stand it. Semi-transparent or not, it’s another car on the track. No matter how many times I’ve tried to convince myself that I can drive right through it, my brain sees it as ‘another car on the track’ and will try to avoid it. So instead of matching my fastest line through the corner, I late apex without thinking so I can dodge the invisible car on the inside.
There used to be a setting that would turn this feature off. After a few hours of searching (It’s not in the manual, or in ‘Settings’.) I found it: Press UP on the Digital pad while driving. The blue dot on the map will go away along with the problems found with racing against yourself.