if you are curious, here is the URL of ELAN who produces the 78p458, you can find a complete datasheet of this micro-controler with 8 I/O - ADC

now, here are my thoughts about the 4 ch and 6 ch transmitter from walkera

you can notice that, on the 4 ch, there is 2 switches and a push button, wich are not plugged, this means that the
"box" is exactly the same for both transmitters.

now, for the board, i wonder this would be cheaper to make a single board for 4 and 6 ch, with the same
micro-controler, same program inside.

so... what difference ? I think that in the 6 ch, the switches and push button are simply plugged somewhere
on the board, and i think that it could be possible to transform your 4ch transmitter in a 6 ch one, simply
by finding out where the switches and button should be plugged on the board, and do it

Of course, this is only an opinion