Miles Davis played so many great tracks that it is hard to say what song is his most notable. "Four" is one of his most popular jam session tunes. It is a simple ABAB 32-bar song form except for an optional tag to the final chorus.
The harmonies aren't terribly difficult once you get settled with the ii-V-I progressions. This is probably the most common chord sequence in all of music. Composers have used this pattern almost since they began writing music. Classical musicians may prefer to call this the sub-dominant, dominant, root sequence or something like that. But the concept of "root" doesn't apply so much in jazz because people like Miles used the ii-V-I pattern as a way to move gracefully between otherwise disconnected key centers.
We find ii-V in the A section in bars 3-5 to get us from Eb major to Db major. In the B section we have the ii-V (of E) with the F#m7 - B7, but in this case, Miles takes us not to E major, but to F minor. It is unexpected, but elegant. The important thing about these patterns is to make sure you land strongly on the chord after the dominant. If it is a true ii-V-I, that is easy because all three chords are tightly related, using the same notes. But if the landing chord is not the expected "root" chord, you must prepare for the surprise landing.
In reality, the song isn't as complicated as all that, and you might do well just to become very familiar with the melody and let your ears guide you.