Externally the main difference is the turbo look body shell - this means the wider rear wheel arches although there are no air intakes in the wings. The rear bumper is wider to accomodate the wider arches and has the exit holes behind the rear wheels. Since there are no intercoolers, they are just cosmetic.
The exhuast tips are the larger size, the same as the turbo.
The distinctive item at the back was the name, with the extra S, and the red reflective strip that ran between the lights harking back to previous 911 generations (no other 996 model has this).
The wheels are turbo II look - but they are not hollow spoke like the turbo, they just look the same. They were 18 inch with 295/30ZR18 rear and 225/40ZR18 front tyres, same as the turbo and wider than the C2. The wheel centres have C4S on them.
The front has the facelifted appearance, since the C4S came out after the facelift.
The front bumper is the same as the turbo and more aggressive, with the wider nostrils at the side and the centre opening for the centre radiator. Unless you have a tiptronic, then there is no radiator in the middle I seem to remember.
The plastic lip on the bottom of the bumper is different to the C2 and different to the turbo.
The door sills have a plastic guard as with the C2, but may say C4S, I can't remember.
The brakes disks are larger than the C2 (same diameter as the turbo) with red 4 pot callipers all round.
The car rides 10mm lower than the C2 - same ride height as the turbo.
Inside the primary difference is that the C4S had a full leather interior, on the C2 this was an option.
Everything should be leather, except for the radio surround in the dash, the centre console between the seats, the storage cubby cover between the seats, the sunvisors and the steering wheel centre.
Electric comfort seats were standard (fore/aft, up/down, back rest angle).
Trip computer was standard.
PSM (stability management) was standard.
Green top tint on the windscreen and floor mats are standard (but this should be the same on the C2s).
Sat nav (PCM), telephone, sunroof, parking sensors, rain sensing wipers, auto dimming mirrors, crested headrests, shortshift, aluminium insert on handbrake and chunky gearlever, CD-autochanger, Litronic headlights, stereo upgrade to Porsche system or Bose were all options on the C4S.