Muliplexer 4-way Implementation proof
It's quite simple.

Since you know how to implement a Mux, consider it as an abstraction. Then on proceed to think of how you would create a 4WayMux out of individual Mux chips.

A Mux converts two inputs into a single output. 

So then to convert four inputs into a single output, we need to combine the output from two individual Mux chips into one single Mux chip.

So input a and b would go into Mux1 and create output1
and input c and d into Mux2 and create output2

Then output1 and output2, would become the input to Mux3, that converts these two inputs into a single output, essentially converting four inputs into a single output.

If you want to construct a truth table around this concept, you'll have to create truth tables for each individual Mux chip involved and combine it, or assume a 4 input Mux chip and create a table.