This way you'll never have 4 new tyres on the vehicle cos if you change only the fronts now they will be still good when the rears are worn and so on, it's not a good ideea at all to mix tyres cos on a long run the transfer box will suffer. Better bite the bullet and fit 4 identical tyres but if you decide to change only on one axle dont fit other dimensions than on the other cos then you'll certainly kill the transfer box and the traction controll will be mixed up as well