Yo Ryan,
This is a bit of reading but....
Dissonant intervals
The perfect fourth is considered dissonant in common practice music when not supported by a lower third or fifth (but see below).
Major and minor seconds, sevenths, and ninths are dissonant. Composer/theorist Vincent Persichetti, in his book Twentieth-Century Harmony, classifies major 2nds, minor 7ths, and major 9ths as "soft dissonances," whereas minor 2nds, major 7ths, and minor 9ths are "sharp dissonances."
The tritone (an augmented fourth or diminished fifth) is dissonant. In the Middle Ages and Renaissance, it was known as diabolus in musica because the perfect fifth was considered to be a reflection of the divine, and the tritone falls just short of a perfect fifth. [Note for the advanced: Technically, it is not proper to refer to the diminished fifth as a "tritone." The word "tritone" refers to "three whole-tones," the distance represented by the tritone. This means that the augmented fourth, which comprises three whole-tones, is a true tritone, while the diminished fifth, because of its accidental-spelling, is not made up of three whole-tones and is therefore not a tritone. However, it is acceptable as an informal convention to refer to the diminished fifth as a "tritone."]
In Jazz, the minor 9th is often considered too dissonant for practical use. This is the basis for some notes being called "avoid notes", typically the 4th of a major scale - it sounds dissonant because it forms a minor 9th with the 3rd. Other "avoid notes" are the minor 6th in aeolian mode, or the minor 2nd in phrygian mode. Some chords are typically voiced to avoid a minor 9th (musicians invert the interval and play a major 7th instead). For example, in a Cadd11 chord (see Complete List of Chord Patterns), there is a minor ninth between the third, E and the eleventh F. If the F is played below the E, the interval becomes a major seventh, which is less dissonant.
The perfect fourth is the inversion of the perfect fifth. In common practice music, it can be both consonant and dissonant: in this case, it has a need for resolution when unsupported by lower notes, in which case it is dissonant even though it sounds as "good" as the fifth. The fourth is always consonant when supported by a lower third or perfect fifth, for example, E-G-C-E is consonant, but G-C-E is dissonant. In more contemporary music, many consider the fourth to always be as consonant as the fifth.
The perfect fourth
In Medieval music, the perfect fourth was even considered a perfect consonance, as the perfect fifth and the octave. However, this attitude no longer prevails.
I hope this helps!! :^D
Black Hawk

My head hurts...

Yo Gerry,
Yeah, I can see that can happen!
Sorry about that! :^D
Black Hawk

When I work, I work hard.
When I sit, I sit loose.
When I think, I fall asleep.

Thinking just isn't my forte, lol.

Yo Gerry,
LMAO!! Well, the Post just under yours explains it a little than mine!! Keep working at it! Your Understanding will come! :^D
Black Hawk

EXAMPLE 1:
Play a C, then play an F one fourth above it in a way that they will ring together. Now play the same C, then play the E just above it in the same manner. If you heard the F as achieving resolution when changed to an E, then you are experiencing this phenomenon first hand. Dissonance is a musical sound that needs resolved, not simply intervals that "sound bad." In common practice music(what most people call classical music) this was only adhered to if the perfect fourth was between the two lowest voices in a chord (i.e. the Bass and Tenor/Baritone).
EXAMPLE 2:
Take a C Major Chord. "C E G".
Now play the F in between the E and G, simultaneously. (You probably need a keyboard to do this, but you can play the 12th fret harmonic on the G string, 15th fret on the D string and the 19th fret on the A string or something equally dangerous)
Notice the clash? The F is too close to the note that defines the tonality of the chord. The E makes that chord sound major. The F fucks with that. In minor chords, the F is okay (4ths or 11ths are kosher in any minor chord) because the F is a whole step away from Eb, the note that defines the tonality.

That's an excellent point you make---about the perfect 4th/11th being consonant (or at least acceptable) with minor chords or triads. I guess I sort of understood that rule in the back of my head, because whenever I would play something in a minor key on the guitar, I was never hesitant to add the 4th somewhere in chord structure, whether I was strumming the chords or "arpeggiating" them... because I knew it wouldn't clash or sound unresolved like it would in a major key.

...just didn't realize why that was, but your explanation makes perfect sense, and I kinda feel stupid for not realizing it myself since it's so obvious haha. Thanks for the response

When you play a triad, the perfect fourth is one half step above the 3rd of the triad, so it produces a dissonance between the 3rd of the key and the 4th. It also produces some sonic confusion as to what is prominent at the time. i.e., are we playing the 4 chord, the 1 chord or a 1 sus chord?

So the interval of a perfect 4th is not dissonant of itself, but just in relation to the 3rd of the key...

It all depends on the register or octave it's played and the voicing of the chord. If for example you play the CEFG chord (note cluster) 2 octaves higher it is pleasing color sound. One of the reasons that chords are expressed as extensions, C11 for example has the F note at the top or Csus4 implies that the 3rd is not played or played in the top of the chord. Then we must also remember the case of CM/F where a C major chord is played over an F in the bass. The 4th is also in most cases a perfectly acceptable passing tone.

are you suggesting that a Csus4 could have a 3rd in it? Wouldn't this eliminates the need sor "sus4" nomenclature. also, C/F does not have a fourth in it. C7/F does. is that what you meant?

are you suggesting that a Csus4 could have a 3rd in it? Wouldn't this eliminates the need for "sus4" nomenclature? also, C/F does not have a fourth in it. C7/F does. is that what you mean

A Csus4 would not have a 3rd, but the 4th in substitute until the resolution. C over F has the 4th in the bass, as does C7 over F, the F of course being the 4th of the root C...

i don't think anyone is suggesting that the 4th degree of a scale is inherently dissonant. Only that the interval of a fourth is. A C/F does not have a fourth in it.
G---\
E----|=C major chord
C---/
F=Bass note

F--->C is a fifth. F--->E is a seventh. F---->G is a ninth.
C-->E is a third. C--->G is a fifth. E--->G is a third.

The interval of a 4th is not always perceived as dissonant, think of the case of a CM chord with the octave included, G-C is a fourth but does not have the came tension with a C in the root position.

that was mentioned twice already in this thread, but your simple way of putting it is very handy indeed.

Yes, more complete explanation. Thanks Brad.