Mnemonics, up and down
Going up: Here Comes the Bride, Wagner's Bridal Chorus, whose opening leap is a perfect 4th. Amazing Grace climbs the same distance, the jump into "a-mazing." Both rise exactly five semitones, so pick whichever melody is already in your head.
Going down: I've Been Working on the Railroad. The opening "I've been" falls a perfect 4th. A falling 4th doesn't sound like a rising one, which is why the buttons above play it ascending, descending, and as the two notes stacked together. Most people can sing the gap once a mnemonic sticks; the harder skill is naming it when you hear it.
Jazz keeps its own reference tunes. Mark Levine's interval table fixes the ascending perfect 4th to Duke Jordan's "Jordu" and the descending one to Wayne Shorter's "ESP."
On the staff
Consonant, dissonant, or both
By the numbers the perfect 4th is about as pure as intervals get: a 4:3 frequency ratio, the third most consonant interval after the octave and the perfect 5th. Played as a bare pair of notes it sounds open and settled.
Harmony complicates that. From the late 15th century (the theorist Johannes Tinctoris pinned it down in 1473), a 4th sitting above the bass came to be treated as a dissonance, something that needs to resolve rather than rest. It's the engine of the 4-3 suspension, where the 4th hangs over a chord and then falls a step to the 3rd, and of the sus4 chord, which holds the 4th in place of the 3rd until it slips down. Medieval singers, for their part, stacked parallel 4ths and called them consonant. The interval never changed; the rules around it did. Kostka and Payne give the modern rule in a line: the perfect 4th is consonant except when it sits above the lowest voice.
Where you'll keep meeting it
Standard guitar tuning is mostly perfect 4ths: E up to A up to D up to G, then a major 3rd to B, then a 4th to the top E. (The violin family goes the other way, tuned in 5ths.) The cadence that ends most pieces moves its bass from the 5th of the key up to the tonic (sol to do), which is a perfect 4th. And every sus4 you've heard ring out at the top of a chorus is this interval, held a moment before it drops home.
What it gets confused with
The real mix-up is the perfect 5th, the 4th's inversion. The two share the same open, hollow color (both are perfect consonances), and that similarity is what trips the ear: the 5th is wider and sounds settled where the 4th sounds slightly unfinished. The tritone is technically the nearest interval, just one semitone wider (six semitones to the 4th's five), but it almost never causes trouble in practice: the 4th is open and stable, the tritone tense and unresolved, and the contrast is hard to miss.
A quick test. If the interval feels like the start of a hymn or a wedding march, like it's going somewhere, it's probably a 4th. If it feels like it's already home, you're hearing a 5th.
How Aubel handles it
The perfect 4th is in the starting pool for every new player, shown on the first morning alarm next to the unison, the perfect 5th, and the octave. The detail screen leads with Here Comes the Bride, since the wedding march is hard to shake, and keeps Amazing Grace as the backup for anyone who knows it better.