From the 996 buyer's guide which is available here
http://www.porscheclubgbforum.com/tm.asp?m=254973
Carrera 4 has 4 wheel drive (most of the time only 5% is transferred to the front wheels but can be up to 40%)In order to make room for the 4 wheel-drive hardware, The C4 had to be re-engineered from the windscreen forward. Front suspension was tilted to the rear to make room for the driveshafts, although under the front boot was reduced from 130 litres to 100 litres.
The C4 weighs 55kg more than the Carrera, but its superior traction off the line means that the C4 is just as quick in a straight line. Weight balance is almost identical to the Carrera, 40/60 front/rear for the C4 compared to 38/62 for the Carrera.
The C4 came as standard with Porsche Stability management (PSM). This traction control system groups together a series of safety devices (including traction control, anti-slip control and Automatic Brake Differential"" ABD) that had in the past worked independently of each other. Traction control now communicates with ABS and a brake differential control to create a system that can brake each wheel individually.
Visually the differences between the Carrera and C4 are limited to "˜Carrera 4' badging, titanium
coloured callipers on the C4 (the Carrera has black items) and black wheel centres on new-design 17-
inch alloys.
In 2000 Porsche also upgraded the Carrera with the C4's drive-by-wire electronic throttle mechanism
(E-Gas) and improved engine management (Motronic 7.2). This makes retrofitting of cruise control a lot easier.
If you don't want/need the benefits of 4 wheel drive (often associated with slightly heavier steering -less feel,more understeer worse fuel economy, and greater tyre wear) in MY2000 PSM became an option on the Carrera, so look out for that if you think you'd like it.
In terms of resale I don't think there is any difference. Purists prefer the regular Carrera for it's rear wheel drive feel, perhaps 'newbies' prefer the added safety and greater all weather useability of the Carrera 4.
Nice options would be heated seats, wind deflector, Stereo upgrade package or Bose, electric memory seats, Litronics,18 inch wheels.
The number one option is the sports exhaust by a country mile, if you can't find one with it get one or a copy of one.