cartography - Legend of a raster map with categorical data

I would like to plot a raster containing 4 different values (1) with a categorical text legend describing the categories such as 2 but with colour boxes.

I've tried using legend such as :

legend( 1,-20,legend = c("land","ocean/lake", "rivers","water bodies"))

but I don't know how to associate one value to the displayed color. Is there a way to retrieve the colour displayed with 'plot' and to use it in the legend?

Initial raster

Raster with legend

Answer

You are not providing a reproducible code, so I made up a simple example that hopefully will allow you to tailor it to your needs.

library(raster)
r <- raster(volcano)
plot(r, legend = FALSE, col = rev(terrain.colors(4)))

legend("topright", legend = c("category 1", "category 2", "category 3", "category 4"), fill = rev(terrain.colors(4)))