I know that Ishigar is the western continent, and Alakitasia is the eastern continent. Does that leave room for a Northern and Southern continent that we haven't heard about or are those two the only ones?
As far as I remember there are currently only two continents defined, Ishgar and Alakitasia. In chapter 439 page 14 there is a map showing the original purpose of the Alvarez empire, which is inhabiting Alakitasia, is to invade Ishgar from over the sea. In this map it shows more land that has not been mentioned before, to the north/north-east of Ishgar and east of Alakitasia, which could be another continent. So yes, this does leave land for possibly another cotenant.
Here is the map that I mention above: