R - adding page numbers to PDF

Question

I'm having trouble adding page numbers to PDFs. Here's how I'm inserting pages / plots:

pdf( file = pdfFilePath , width = 11 , height = 8.5  )
for ( ... ) {
    grid.newpage()
    pushViewport( viewport( layout = grid.layout( 2 , 2 ) ) )
    ... print 4 plots ....
}

onefile seems to name a file by the page number, but I want the page numbers to appear in the same file.

Edit
I've modified @Gavin's code sample to produce a working version of mixing graphic types to get page numbers:

require(ggplot2)
pdf( file = "FILE_PATH_TO_SAVE_PDF_HERE" , width = 11 , height = 8.5  )
par( oma = c ( 4 , 4 , 4 , 4 ) , mar=c( 4 , 0 , 2 , 0 )  )
plot( 0:11 , type = "n", xaxt="n", yaxt="n", bty="n", xlab = "", ylab = ""  )
mtext( side = 3 , line = 0 , outer = TRUE  , cex = 1.5 , family="mono" , "Title" )
grid.newpage()
p1 <- ggplot(data.frame(X = 1:10, Y = runif(10)), aes(x = X, y = Y)) + 
        geom_point()
vplayout <- function(x, y) {
    viewport(layout.pos.row = x, layout.pos.col = y)
}
pushViewport(viewport(layout = grid.layout(2, 2)))
print(p1, vp = vplayout(1,1))
print(p1, vp = vplayout(1,2))
print(p1, vp = vplayout(2,1))
print(p1, vp = vplayout(2,2))
mtext( "1" , side = 1 , line = 3 , outer = TRUE , cex = .8 , family="mono"  )
dev.off()

Answer 1

From my second comment on the Q, I suggested a base graphics solution using mtext(). This appears to work for the OP so I show an expanded version here:

Base Graphics:

op <- par(oma = c(2,0,0,0))
layout(matrix(1:4, ncol = 2))
plot(1:10)
plot(1:10)
plot(1:10)
plot(1:10)
mtext(side = 1, text = "Page 1", outer = TRUE)
layout(1)
par(op)

Resulting in:

@SFun28 reports this idea works for his ggplot/grid graphics too, but it does not for me. After running the last line of the code chunk below I get the following error:

> mtext(side = 1, text = "Page 1")
Error in mtext(side = 1, text = "Page 1") : 
  plot.new has not been called yet

which is indicative of the warning not to mix base and grid graphics.

require(ggplot2)
p1 <- ggplot(data.frame(X = 1:10, Y = runif(10)), aes(x = X, y = Y)) + 
        geom_point()
vplayout <- function(x, y) {
    viewport(layout.pos.row = x, layout.pos.col = y)
}
grid.newpage()
pushViewport(viewport(layout = grid.layout(2, 2)))
print(p1, vp = vplayout(1,1))
print(p1, vp = vplayout(1,2))
print(p1, vp = vplayout(2,1))
print(p1, vp = vplayout(2,2))
mtext(side = 1, text = "Page 1")