The metro population is 4.5 million. If you're not counting the metro populations, then you need to drop Porto Alegre, San Francisco, and Belo Horizonte.
We don't! In fact citypopulation.de doesn't say the "real" name is Saigon at all, and they don't claim Ho Chi Minh City is the other name either. They say the "name" is Thành Phố Hồ Chí Minh (5 words now!!!) and the "English name" is Saigon, and we don't think either is an accurate reflection of its standard name, and we're happy to set the quiz to reflect that.
If you insist on citypopulation.de being authoritative, then you're missing "Kuwait City", "Cape Town", "The Ruhr", and "Cologne / Dusseldorf".
And (according to them) the "real" name for Ho Chi Minh City is "Saigon"!