It is a combinational circuit which have many data inputs and single outputs depending on control or select input. We can say that for 2^n input lines, n selector lines are required. Multiplexers are mainly used to increase amount of the data that can be sent over the network within certain amount of time and bandwidth.
In 4X1 multiplexers, there will be 4 inputs, one output and two selection lines. Let's us suppose we have four inputs i.e. A,B,C,D and two selection lines i.e. S1 and S2 and one output i.e. Y.
So, after implementing the truth table we got -
Y = S1' S2' A + S1' S2 B + S0 S1' C + S0 S1 D
And then the combination circuit will look like this -