ALL Discoveries are permanent 4WD. The little gearstick, if you move forwards & back, selects low or high ratio and if you move it to the left it locks the transfer box diff - this is why the sign illuminates on the dashboard. It is there to remind you to take the vehicle out of difflock as soon as the conditions allow. You shouldn't be on a firm surface, ie road, with difflock on as you will get transmission wind-up (which is bad). Difflock is an extreme off-road tool and only 'proper' 4WD trucks have it. For typical off-road conditions you won't even need to use it. All Defenders are permanent 4WD too, it's only the old Series Land Rovers that were selectable. They were either rear wheel drive or 4WD, depending on the levers engaged in the cab. Get yourself an owner's manual from a dealer or off Ebay.