Imagine it this way.
Civilization 4 is a mature adult with deep aspects to his thoughts and personal nature, while knowing what he wants to become and achieve with his life. Overall, Civ 4 has become a 40 year old man, who is both accomplished over many years of work, and mentally stable due to immense support from his extended family.
While...
Civilization 5 is a somewhat promising child of 10, who is sometimes overactive, unattentive of his homework, and has quite a bit of a persistency problem. He has not thought much of what he may or will become once the years take their due, but he persists in shallow horseplay, performing great physical spectacles only the young are capable of, all the while unaware of greater problems looming on the horizon of time. He may grow up to be an unthinking doll of an idol pop star, or focused and simple sports phenomenon, but on the other side, maybe he'll become a greater man than his predecessor, taking the mantle of greatest civ sim of all time, and starting a new age of Civ fandom.
Do I marry Civilization 4 or abandon Civ 5 for adoption? You know what. I propose to keep both.