How does an 8-4 multiplexer function?
Another way of stating a 8-4 multiplexor is as four copies of a 2-1 multiplexor. The SELECT pin connects to all multiplexors, so they all choose the '0' or '1' input together. This allows you to switch 4-bit buses relatively easily with few pins.



The datasheet is a little more confusing because it includes the discrete logic version of a multiplexor. They probably did that so that the tri-state output can be shown un-ambiguously.