Adding multiple analog signal
This is a little unclear what you are actually having problems with, but I'll try:


  Is adding signals are not commonly used?!why? Are everybody uses the Op-Amp method?


It's very common, and it's so common that an IC was designed to do it: The operational amplifier, which can do operations such as summation - exactly what you want! How to do so is explained in one of the answers you linked to.


  multiplex at 4xbandwidth of signal, then LPF to 1/4, tada, the answer will be average of all 4


Well... yes, but you would need a very good filter for this to work, in combination with multiplexing at a much higher frequency. It is not really useful compared to a simple OP-amp adder.