How to get single value of NDVI value from four different directions?
After given some information about the data source I have a simple answer. Since the underscores SW etc. represent the four pixels around the city center and all the pixels cover the same physical area, taking a mean of the values will represent the average NDVI of the total area.

It is all about what you will do with the NDVI data. E.g. if something interesting occurs when somewhere near the center vegetation amounts are low (high), then the minimum (maximum) of these might provide you with more useful predictor.