if you speakers weigh 100 lbs.
4 wheels = 25lbs/wheel
3 wheels = 33 lbs/w. that's + 1/3 MORE weight/wheel, thus less likely to move/vibrate.
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any floor, even a hard flat one, can have imperfections from dead flat.
to avoid shims, even very thin ones, to keep weight equally and heavier on the 3 wheels, to keep the positioning you chose: use 3!!!!!!