I have all versions including 2018 one, all of them have 4 speakers but the 2015 has the best sound quality between all models from 2015 to 2018.
It has the most balanced sound, the best high and mid ranges, the low register is also sounds better even if it has less low frequencies.
As for 2017 version. It's 5-7% louder than the 2015 version but its mid and high ranges are less detailed (not pleasant to hear comparing to the 2015, even w/o comparing it's obvious that something is missing, the low end on the contrary is too loud and bulky that ruins the whole balance. Also it tends to overload the sound volume so it can give you a harsh/cracking sound, so I'm disappointed in this regard.
As for the 2018 version (12.9), again it sounds like from the closed barrel, does not have enough mid and high range, the low range became even more punchier than the 2017 one, not pleasant. It's very loud but not balanced at all. the 11 inch version has a better balanced sound than the 12.9 version although it's not that loud.
For example if you record a voice (thru ext Cardioid Condenser Mic and listen it on 2015 version it sounds almost identical to the original) but both latter versions lack of high and upper mid ranges, so your voice sounds like it was recorded with some foam rubber placed between your mouth and the mic.
Another example in Garage Band you need to add about 15% to treble in order to get similar sound in upper range.