On sale from: 1st September 2009

Price: from £31,995

Engine: 3.0 litre V6 turbodiesel, 245 horsepower, 600 Newton metres of torque

Acceleration: 0-60 mph in 9.0 seconds

Fuel consumption: 30.4 mpg (combined cycle)

CO2 emissions: 244 g/km

Rivals: Mercedes ML, Volkswagen Touran

The first Land Rover Discovery burst on to the scene twenty years ago. Before that, if you wanted to buy a Land Rover, you had a choice between the plush, expensive Range Rover – if you could afford it - and the Defender, an immensely capable off-roader, but one that had the sort of stripped out interior you'd expect in a vehicle selling in large numbers to armies and farmers. Land Rover, which along with Jeep had pioneered 4x4s, didn't have a modern, practical SUV that could appeal to families, and had therefore ceded a certain amount of market territory to newcomers such as Mitsubishi's Shogun. These cars didn't have a Land Rover's off-road pedigree but most of the people who bought them probably didn't care.

The first Disco, as it quickly became affectionately known, changed all that. It was an instant hit, and its fresh styling and Conran-designed interior concealed the fact that it was actually a crafty adaptation of the 1970 Range Rover as far as the bits under the skin were concerned – although these were still competitive by the standards of the time.

The second-generation Discovery of 1998 was really just a heavy update of the original but the big milestone in the Disco's history was the introduction in 2004 of the Discovery 3. This was a clean-sheet model, funded during Land Rover's period of ownership by Ford. In fact, the Discovery 3 was a technical tour de force; a new form of construction, which combined the advantages of a separate chassis and an “all-in one” monocoque body, all-round independent air suspension, a six-speed automatic gearbox from ZF, and an excellent new V6 diesel engine – it had them all. But the most impressive piece of new technology was “Terrain Response”, which made the Discovery's huge off-road capabilities more easily accessible to less experienced drivers. The system is controlled via a simple dial on which a setting corresponding to prevailing conditions (e.g. “Sand” or “Mud and Ruts”) is selected. The car then automatically selects, for example, the appropriate ride height, high or low range settings on the gearbox, and so on. The system can even soften the car's throttle response if that is appropriate for the type of terrain being covered. The result of this effort was that Disco 3 was an outstanding off-roader and a superbly comfortable car for long road journeys, as well as providing a passable driving experience.

The new Discovery 4 is a heavy reworking of its impressive predecessor, rather than an all-new model. A chance to try the car over difficult terrain in the Scottish borders, climbing and descending extreme muddy slopes, fording streams and rivers flowing across loose, rocky surfaces, and negotiating just about every other obstacle imaginable, showed that there have been further improvements to the car's excellent off-road ability. Passengers get an even better deal than before, thanks to a revamped interior and improved levels of equipment.

But the main advances this time around are in the Discovery's abilities as an on-road driving machine. The V6 diesel engine, which had a capacity of 2.7 litres in the previous version, has been increased in size to three litres and also benefits from a series of other modifications, measures that boost power by 29% and torque by 36%, while reducing CO2 emissions and fuel consumption at the same time. The result is that the Discovery, a heavy car, now feels much more lively than it did before. You don't need a stopwatch to measure the changes – they are immediately apparent. Suspension and steering changes also noticeably enhance the Discovery's cornering abilities, and the braking system has been uprated too.

As well as its fresh interior, the Discovery 4 receives a number of external design changes. There are fewer unpainted plastic mouldings and the Disco's rather brutal squarish looks have been softened a little, especially at the nose, with a bit of subtle recontouring. Much of the car's detailing (headlamps, tail-lamps, grille, and so on) has been enhanced in the interests of Land Rover's continuing pursuit of “premiumness”, a ghastly word but one that captures precisely the improvements in perceived quality the company is trying to make to all its products - although making the Discovery classier may just make it more difficult for Land Rover to persuade customers to cough up the extra cash for the similarly sized but ostensibly posher Range Rover and Range Rover Sport. We shall see.

The mood of the times may be against the Discovery 4, its bulk, its weight and its superb four-wheel drive system that is unlikely to be exploited to the full by most of the people who buy it. But it is an outstanding product.

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