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Much smaller CPU, less need fo heat dissipation. The iFixit teardown goes into it a bit.

ATV uses passive heat dissipation, nobody wants noisy boxes in the entertainment area to disrupt the sound.

The 4K model actually does use a fan for cooling. The 10X processor probably puts out quite a bit of heat.
 
IIRC, the ATV2 has a large heat sink for heat dissipation, and the ATV3 did not due to the A5 (or whatever was in the ATV3) being much more efficient.

Although, the ATV2 heat sink still fit in the same sized shell of the 3.
 
After getting use to use to it, I hardly notice it anymore (the difference between the 3 & 4)
 
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