4 to 1 multi-bit multiplexer implementation [Q]
A 4-input mux has 4 data inputs and 2 address inputs. The address inputs determine which data input connects to the output. A 4-bit, 4-input mux is simply 4 each 4 input muxes in parallel, with the address inputs connected the same to all muxes. So, if you have 4 data channels, (A0,A1,A3,A3), (B0,B1,B2,B3), (C0,C1,C2,C3) and (D0,D1,D2,D3), and address of 0 will produce 4 outputs: A0,B0,C0 and D0.

Drawing the entire unit as a block will have an input side (usually shown on the left). and an output side. The input side will have 18 inputs - 16 data and 2 address. The output side will have 4 data outputs.