The lifespan will depend a lot on the model of 4x4, the kind of four-wheel-drive system it has and what tyres are fitted. Generally, front tyres wear quicker than rear tyres as they're used for cornering. This isn't the case for rear-wheel-drive cars, which can often wear out the rear tyres quicker. Most modern four-wheel-drives aren't four-wheel-drive all of the time. They're generally front-wheel-drive under day-to-day driving, sending power to the rear axle when required. As well as being more frugal, this usually means they'll wear out their tyres at the same rate as an equivalent two-wheel-drive model. Obviously, if you do a lot of off-roading or regularly drive in slippery conditions, when four-wheel drive is required, the rear tyres may start to wear out sooner. It's a good idea to rotate the tyres between the two axles if they're wearing at different rates.