Charlotte is a not a very walkable city but there is a strong consumer preference for areas that have establishments (bars, restaurants, tennis courts, parks, etc.) or public transportation nearby and within walking distance. If you can find area that are becoming more walkable like areas near Noda, you can do well. Or if you can buy in area, where an older apartment complex is being replaced by mixed use buildings (residential and commercial establishments) - Silver Oaks being replaced by the Meredian for example, you can do well. Or where a new light rail station is being placed, you can do well. Plenty of other ways to make money as well.
There are not a lot of 4plexes in Charlotte but its a great city for rental real estate. More so then the others you mentioned, I think. You will have a difficult time find fourplexes here though. They are not that common in Charlotte.
Denver has been hot but runs a regulatory risk - if it becomes easier for marijuana related business proceeds to be deposited at banks you could stop see a collapse in part of whats currently proping up the real estate market.
There is a lot of opportunity in Detroit but watch out for contingent liabilities from code enforcement issues on deep discount property.