Due to the geography of the earth as well as other factors, the effects and experience of sea level rise is not the same everywhere. At this moment the sea level at the port of Miami is 8 inches about the 1950 level, significantly more than 3-4mm a year and the rate is accelerating.

Also keep in mind the sea does not need to permanently cover land to make it unlivable. It's well within the realm of possibility that due to expanded floodplains, increased intensity and frequency of storms, that portions of Miami could be unlivable in as little as 30 years, and the entire city unlivable by the end of the century. There's virtually no chance that all the populated areas on the costs will remain livable in the same timeframe.