As pointed out in another comment, it really is 4MB, since the segment register the memory is accessed through is exposed on the bus. You could include those e

As pointed out in another comment, it really is 4MB, since the segment register the memory is accessed through is exposed on the bus. You could include those extra 2 bits for a 22-bit total address space, although I don't think it's really practical to have such a partitioned memory model for a general purpose computer.