I checked the ground clearance on my 1999 +4 when I first bought it about 8 years ago, as I suspected it was considerably lower than the 100mm stated in the owner's manual. It turned out to be 70mm. (This was at the gearbox cross member). So it was no wonder the car became intimate with speed humps and other road irregularities.
It was due new tyres a while ago, so I took Gambalunga's advice and replaced the original 195/60/15's with 195/65/15's, which gave me a welcome 10mm extra ground clearance - and coincidentally gave a more accurate speedo too.
However, I've gained a very welcome further 20mm clearance by shortening the rebound springs by 20mm (at zero cost), which has also improved the ride quality considerably. So it might be worth checking if this mod would work for you.
(Check to see if you can rotate the rebound springs without too much effort with the car standing on the ground as normal. If you can, then this mod wouldn't give any benefit, but if you can't rotate the springs then it implies the springs are being unduly compressed at rest and could be shortened).
So I now have 105mm clearance at the gearbox cross member, and 124mm at the rear cross member just to the fore of the back axle.
