1:4 Demultiplexer Structure

module demux_12(a,s,c);
  input a;
  input s;
  output [1:0]c;
  assign c[1] = (a & ~s);
  assign c[0] = (a & s);
endmodule

module demux_4to1(a,s,c);
  input a;
  input[1:0]s;
  output [3:0] c;
  wire [1:0] c1;

  demux_12 intermediate(.a(a),.s(~s[1]),.c(c1));
  demux_12 out (.a(c1[0]),.s(~s[0]),.c(c[1:0]));
  demux_12 out2(.a(c1[1]),.s(~s[0]),.c(c[3:2]));


endmodule