It depends. A 4" tube running the length of the vehicle would just waste space. Too much skin friction.
Modern practice has an envelope body that is hollowed out. MacLaren F1 and others. They try to get the air out behind the front wheels, then use a flat floor to set up for a difusser.
See the size of that duct? There're two of them. Here's a short duct on a Ferrari:
It's purpose is to move air away from the A-pillar. See also Viktor Schauberger's trout drive submarine:
It exhausts into a Coanda nozzle at maximum body cross section.