One more chord finishes the starter kit: 6m — in G that's Em. It's Home's minor twin — same family, same notes, but wistful instead of bright. Now you hold four chords, 1, 4, 5, 6m, and four chords is a whole songbook.
H = Home · A = Away · T = the Question (leans Home) · h = sad cousin
Try this now Play G, Em, C, D around and around. Hear how the Em darkens the room without ever leaving the key? That's the sad cousin doing its quiet work.