To understand what is meant by the terms 2-pole filter and 4-pole filter, you first need to know that the word "pole" in the context of filters refers to a 6 dB per octave roll-off.
Well, a 2-pole filter = 12 dB/octave slope, and that's because 6 dB x 2 = 12 dB. Furthermore, a 4-pole filter =...