R: Adding a text box fourth panel of a grid populated by three charts

R: Adding a text box fourth panel of a grid populated by three charts

I can create a 4*4 grid and populate it with three charts (example from
the literature):
# 3 plots arranged in 2 rows and 2 columns
attach(mtcars)
par(mfrow=c(2,2))
plot(wt,mpg, main="Scatterplot of wt vs. mpg")
plot(wt,disp, main="Scatterplot of wt vs disp")
boxplot(wt, main="Boxplot of wt")
But how do I create a panel with just text and then add it to the fourth
position in the grid.
I would be grateful for your help.