Why 4 bit address bus is grounded?

The data bus in the 8086 is 16 bits in size, while the address bus is 20 (16bits would only address 64KB of memory, an extra 4 bits allows to address the total of 1MB, this is done trough segmentation of the memory).

To form a multiplexed of data bus and address bus, four bits of 8086 address bus are grounded.