What is the perceived output frequency (Audio) when 4Hz and 8Hz are played together?

Answer 1

IF they are precisely in phase (zero crossing of the 4Hz is always held in phase with the 8Hz crossings), there would be an reinforcement of amplitude at the 4Hz peaks as the two intersect. This occurs only because 8 happens to be a multiple of 4. If you'd picked 5 and 8, things would be a bunch more interesting.

However, while you do say "audio", the perception wouldn't be by ear -- you can't actually hear anything at frequencies that low. The bottom limit is pretty universally accepted as 20Hz. With two in-phase signals at 4 and 8Hz, you'd feel it, but probably not be able to tell that anything was happening frequency-wise apart from the sense of vibration.
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Answer 2

Also assuming it's 4 and 8 kHz, what you perceive is 4 kHz with what's called "harmonic distortion". 8 kHz is the 2nd harmonic of 4 kHz. Being twice the frequency, it's an octave above 4 kHz. Even-harmonic distortion (2x, 4x, etc) frequently occurs in musical instruments and in small amounts (a few percent) is hard to detect; at higher levels it is not too objectionable even when unnatural, i.e., added by a distorting amplifier. (Eventually, of course, if the 8 kHz amplitude becomes comparable to that of the 4 kHz tone, you will perceive 8 kHz as an independent tone.) On the other hand, odd harmonics (3x, 5x, etc) are easier to notice and generally more objectionable.
BTW, answer 1 erred in saying the waves will reinforce at the 4 Hz peaks if their zero crossings coincide. Zero crossings occur each 180 degrees of a given wave; the 8 Hz zero crossings will occur every 90 degrees of the 4 Hz wave, or at its zero crossings and peaks. This is not reinforcement.

Answer 3

The perceived output frequency of anything
at 4 hz is a gigantic woofer collapsing on a stage.
So it's not really perceived so much as fallen through.

Answer 4

Lets consider 4kHz and 8kHz (4Hz and 8Hz are inaudible).
You will perceive 4kHz note. 8kHz will be perceived as
its overtone.

If you feed the output into spectrum analyzer, however,
it will perceive them for what they really are:
4kHz and 8kHz peaks.