I can't say much about the comparison of the three games, because I haven't played Civ IV, and I got to Civ V when all the DLCs and expansions were released, so I can't judge how good/bad it was in Vanilla. But if I can say anything, Civ V music* and leaderscreens beat the other two very easily, IMOMaybe if it had been V > VI > IV, but there is no way V is even remotely close to either IV or VI in quality.

*Except for opening themes. Terra Novum is leagues below Rise and Fall, Sogno di Volare and Baba Yetu.