the 4.0 was probably mated to a zf 22 trans from factory, the 4.6 was mated to a zf24 due its higher torque and rpm curve.
the 4.6 will bolt onto the zf22 with out modifications. however, if your zf22 has not been maintained well or it is of high mileage, it is recommended you be easy with the throttle as 4.6's will devour zf22's. btw you can retrofit a zf24.
it is not possible to tell how long a zf 22 may last when mated to a 4.6. your current ecu will work your 4.6 but if you have access to land rover diagnostics it is advisable you change your ecu profile from a 4.0 to a 4.6 in order to take advantage of the 4.6 fueling and performance mapping.
otherwise enjoy the extra output and performance.
I did this last year. The same trans was fitted to all the later models, the 4.6 did have a slightly bigger oil cooler but I think unless you are crossing the sahara all will be well. I put every thing electronic off the 4.0 into the 4.6 so the car did not know anything had happened (even the inlet with all injectors).
If you can get access to a multifunction diagnostic tool it is worth having the gearbox ECU switched to 4.6 (only takes minutes) I've not managed to find one yet to switch the engine ECU but general consensus is that it is not worth it