Whether a song is traditionally sung with an accompaniment or not, all melodies imply chords and can be put to an accompaniment or arranged for musical ensemble (see Vaughan Williams, Delius, others).

The melody dictates the chords, keeping in mind, of course, that there are often choice points where either of two or more chords may appear to be perfectly correct. At these points, the context will usually determine which one to use.

The change from IV to I is called the "plagal cadence" or sometimes the "church cadence," as in the "Amen" at the end of a hymn. The change from V to I, or especially V7 to I, is called the "authentic cadence." It's stronger, and implies a necessity to return to the Tonic (the I chord). The interval of a diminished 5th in the V7 chord is dissonant, and it demands resolution. This is why a V7 is sometimes called the "drop the other shoe" chord.

Don Firth