I have a song I wrote and at one part it uses the following chords

B Major, E minor, C major, G major

When trying to determine the key of this song, I can only come up with E minor but from researching that doesn't contain a B major chord. (though all the notes of a B major are part of the e minor scale) As you can tell, I'm a beginner when it comes to theory.

These chords sound good to my ears but is it "musically unsound?"

I'm not familiar with music theory, though I'd like to be, I'm hoping someone can help me with this specific question.

All of these chords are present in the E harmonic minor scale, which is a very common classical mode. Your question however should be redirected to the player forum or adult beginner forum. This forum is aimed more at buying, selling and maintaining pianos.

Yes, this is not the right forum, but in the meantime...

It IS the E harmonic minor scale. The E harmonic minor scale is the one we use to get the harmonies (duh!) out in classical music. And the harmonic scale, has the 7th note (the D) augmented a semitone (from D to D#), so this is why you get a B major rather than a B minor...

Sorry--no. G Major is not part of the harmonic E minor scale (the third degree of the harmonic minor is an augmented chord). If you have a G major and a B major in the same song section, I would be looking for a melodic minor (will be B major ascending, but G major descending).

Bob

Whether one uses the harmonic minor or natural or melodic minor is determined by the chord progression of the moment. All are forms of the minor scale. G major has an important function in the key of E minor that of relative major. So to say a G major chord doesn't exist in the E harmonic minor scale, while correct, ignores the fact that you wouldn't normally play a harmonic minor scale while playing a g major chord in the key of E minor. B or it's equivalent B7 serves as the dominant of E minor by incorporating the leading tone (D#) which is available in the harmonic or ascending melodic minor scales. A G chord would then cause a player to shift to natural minor or the descending form of the melodic minor scale.

The thing about minor keys is that the accidentals found in the harmonic or melodic forms of the scale may or may not be used. Although chord V in a minor key is usually major because of the raised 7th in the harmonic scale.

Sounds to me like you are in E minor.

Think of a song like 'The House of the Rising Sun'...

E minor, G Major, A Major, C Major
E minor, G Major, B Major........

Well now I'm confused. Seems to be a dispute here of which key it is in. On another note, where would you recommend a beginner to start learning about theory?

I would recommend a teacher if you have the funds. You can also learn about this online--it's a huge subject and as you can see, can get very complex and technical. While composing can be done by ear well, I think good musicians will strive to accurately understand the theory and notation.

People can do all kinds of things within this key such as passing chords, secondary dominants, non-harmonic tones, progressions that will alter the strict key, and so on, but there should be no dispute that notationally the only possible standard key that will fit those chords you list is the melodic E minor key.

Bob

As I tried to make clear the key is e minor but the scale varies by what the chord is at the moment. You can say the melody determines the chord or the chord determines the melody, but the scale used (the melody) needs to match the chord in the accompaniment. My point is there is no such thing as the melodic E minor key. The melodic minor scale can be used depending on the melodic motion and underlying harmony, but the key doesn't vary based on what scale is being used. Sorry don't mean to get technical, but wanted to clear up a potential misunderstanding.

What chord do you start with, and more importantly finish with? Nearly all music will resolve to its home key in the end.

I'm going to speculate a bit here.
I assumed you are starting your progression from a B maj in a root position, then 2nd inver. Emin, then C major in root position then the questionable chord "G major". You are in the key of E minor and in reality you are placing an E min chord and you are adding a 7 while leaving the root out, this is why harmonically works, in a way you are resolving in the right key. (IMO it is not a G major)(You probably have the root in the melody)

Play all the chords in root position listen and pay attention to G major, play it again but replace it with E min. See which one works better with your melody.

Knowing some of the chords in a song without knowing what order they're played, context, etc etc. doesn't enable us to make the call. Why don't you post a link to a recording of it?

its a traditional e minor chord progression.

4 chords at any one particular point of a song can't really on its own determine the key, unless they are the only 4 chords in the song.

It would be helpful if you posted the chord structure of the entire piece to provide some context (what comes before/after the part you posted, as well as beginning and ending progressions of the piece).

Or better yet, post a recording of the piece in question...would love to hear it!