overlap monocoloured raster layers using level plot in R

Question

I am trying to overlap four raster layers using level plot, I seem to have it all good but the raster just does not show up in the graph, any hint?

 require(raster)
 library(rasterVis)
 r <- raster(nrows=10, ncols=10)
 p = CRS("+proj=longlat +datum=WGS84")

 r <- setValues(r, 1:ncell(r))
 projection(r)=p
 r2 <- setValues(r, 1:ncell(r)*2)
 projection(r2)=p
 r3 <- setValues(r, 1:ncell(r)*3)
 projection(r3)=p
 r4 <- setValues(r, 1:ncell(r)*4)
 projection(r4)=p

  plotOptions <- list(panel.aspect = 1, layout = c(1, 1))
  levelplot(r, margin = FALSE, par.settings = rasterTheme, 
      main = "Overplotting Four r Layers", 
      interpolate = FALSE, 
      at = seq(0, 1, length = 100), 
      panel = function(x, y, z, ...) {
        # Overplot the other three r layers
        plot(r2, add = TRUE, col.regions = "red")
        plot(r3, add = TRUE, col.regions = "green")
        plot(r4, add = TRUE, col.regions = "blue")
        # Set the axis limits to match the first r layer
        xlim(extent(r)[1:2])
        ylim(extent(r)[3:4])
      }, 
      par.strip.text = list(cex = 1.2), 
      scales = list(draw = TRUE), 
      colorkey = FALSE, 
      ... = plotOptions)

Thanks in advance,