You definitely need 4wd..

You might get along with a regular car if:
1. It doesn't rain AT ALL.
2. You are staying close to town and do not plan to drive around the island.

Otherwise you need 4WD.

We got a regular car and let me tell you.....that was a mistake. Go 4WD with a high clearance.

We always take a car (Toyota) over and haven't had any problems. If you are driving to some of the villas definitely go 4 wd.

Below is 7 videos of the North Shore Road from Mongoose Junction to the Smoothie stand at the intersection of North Shore and Centerline. Although it is difficult to judge the grade from a cam on a moving jeep it will give you some idea of what to expect. If you want to jump right to the steeper, hairpin turn video click on the 4th link. It starts just past Trunk I believe.
http://www.on-stjohn.com/2007/10/26/jeep-cam-north-shore-road-part-1/ 1
http://www.on-stjohn.com/2007/11/02/jeep-cam-north-shore-road-part-2/ 2
http://www.on-stjohn.com/2007/11/19/jeep-cam-north-shore-road-part-3/ 3
http://www.on-stjohn.com/2007/12/03/jeep-cam-north-shore-road-part-4/ 4
http://www.on-stjohn.com/2007/12/14/jeep-cam-north-shore-road-part-5/ 5
http://www.on-stjohn.com/2007/12/24/jeep-cam-north-shore-road-part-6/ 6
http://www.on-stjohn.com/2007/12/31/jeep-cam-last-stretch-of-north-shore-road/ 7

That hairpin turn just past Trunk is something special. About 3 years ago a light rain had just started when we attempted to go up that turn.
We were in a jeep Wrangler. The back wheels started to fishtail and we ultimately started to slide back down the turn with the rear slipping around. I slammed on the brakes turned around in the middle of the road and went back to Hawksnest ....... You definitely need 4 WD on STJ.!

Part of the fun of visiting STJ is the driving . . . hills, blind curves and donkeys all while navigating the left side of the road. And if there is a shower it gets slippery. So let your hair down, or fly in the breeze, and definitely go with a jeep, 4WD of course.