How do I specify the color of ggmaps points based on column data?

Question

I have a data frame such that each row contains location data (latitude and longitude) and categorical integer numeric values ranging from 1-4.

Something like this, but with more meaningful lat/lon values:

id   | lat  | lon  | cluster
---- | ---- | ---- | -------
1    | 12   | -32  | 3
2    | 15   | -36  | 1
3    | 16   | -37  | 3
4    | 11   | -37  | 1

I have plotted the points on a map, but I want to be able to specify the colors of the points. Can I specify, for example, that points belonging to cluster 1 appear red, points belonging to cluster 2 appear orange, points belonging to cluster 3 appear green, etc.?

m<-get_map(location=c(lon=0, lat=0), zoom=1)
ggmap(m, extent = "normal") + geom_point(aes(x = lon, y = lat, color = cluster), data = df)

I assume there's something I need to add to the aes() function of ggmap, but I can't figure it out.

Thanks!