I had my Jap import E89 converted to 'normal' satnav reception by the car dealer before it was delivered to me, so I'm not sure what they do to make it compatible. I know that Japanese satnav works via land-based antennas (as a lot of cars in japanese cities can't 'see' GPS satellites because of the tall buildings around) so it's not just a straight swap of units, unless the Z4 comes with the satellite aerials pre-installed? (kind of like how the pre-facelift E85s came with the wiring in place for all the options whether you specced them or not).
Probably not a very helpful answer sorry