I am in the US and went to Sony’s bootloader unlock page and they say that the US variants of the 1 iv and the 5 iv cannot be unlocked. It doesn’t mention other models until you enter your model from the dropdown list.
INTERESTING NOTE! For anyone interested in the new xperia 1 vi, selecting it from the dropdown list brings up a note that the US and JP variants cannot be bootloader unlocked. Does this mean the 1 vi is coming to the US after all?