Your friendly 4 chord is the exotic Neapolitan of the key a major third up. Enter it as furniture, exit it as drama: the classical route to a chromatic-mediant destination.
hear it in Schubert songs · dramatic art-music pivots
Lead-in — how players set it upUse 4 in a totally ordinary way first — 1 → 4, nothing suspicious.Or darken 4 to 4- first — a borrowed hinge before the exotic one.Or sit on 4 as a vamp so its re-badging lands as a twist.Follow — where it usually goesFrom new 1, the fresh key's own 4 confirms you've moved.The new key can Neapolitan right back — mirror door, mirror exit.Or stay and mediant onward — you're already in strange country.
Two wires cross this border: hold the shared 4 across the b2 chord, then step your line down into the new key's leading tone.Target tones (in C)F — lean on A (3)B7 — lean on D# (3) · A (b7)E — lean on G# (3)Half-step wiresF → F# · C ↘ B · D# → E · A ↘ G#Across the borderhold these into E: E