Assuming you were roaming with ATT in the US:
What ATT in the US calls 4G is NOT the same 4G that Vodafone will call its LTE network when operational in Italy. So that's why you automatically "got" 4G when you landed in the US.
Besides the "4G" network that you got in the US, the US has its own LTE network which is incompatible with the European LTE network.
The only time an iPhone displays "4G" is in the US on the AT&T network. When it is displaying 4G it is actually connecting to the HSPA+ network, which is 4G.
HSPA+ is NOT LTE.
As has been stated, the A1429 GSM iPhone sold outside the US will not work on the AT&T LTE network and would be unlikely to work on any other US LTE network.
Seems you may still be crazy.
i was merely trying to help since it was such a nightmare for me.
thanks for the additional info, i have the A1429 - am i correct to assume it is the GSM model and not the CDMA model? it looks like the LTE bands here (1,3,5) are different than the ones in the A1428 model (4,17) - is that why it will not work on the US LTE network? I dont really care "that much" because I live in Italy (for the next 3 years), and do not plan on traveling to the US that often. plus ive had 3G forever, so anything faster is an immediate improvement.