How to combine demux units and other circuit components in order to obtain a bigger demux?
for 4-16 use the Most Significant I/P line as a select line to separate two  3 to 8 decoders. When it is 0 one of lower significance is enabled when 1 then other. For 74HC238 used as lower decoder take three dummy lines with logic 1 and "AND" two them with most significant I/P (here selection line say s1) to form connection of not(E1) and not(E2) and use "nand" with E3 to form  logic.

Note that when select line is high it implies decoder with higher significance is activated and vica versa is also true.

Similar procedure may be followed with the decoder with higher significance.