Someone raised the question if the number of control wires could be reduced from 5 to 4. 

Yes, this is possible with a simple external circuit with only 2 gates:

The new control wires are named M0 - M3. This is the new function table:

If one of these 9 functions is dropped, it would be possible to have only 3 control wires. But I don't think the conversion will be as simple as from 5 to 4.

But if the Exor-gates, that can invert the A lines, are removed, then also the M0/F0 control line can be removed, leaving only 3 control lines. The functions B - A, XNOR and NOT A will not be available in that case. But you can still invert a value by XOR'ing with 0xFF.