Changing default order of plots in par

Hi,

I wanted to change the order of how the plots appear in a multiplot
scenario. For example, in the code below:

#####

pdf('test.pdf',width=8,height=8)
par(mfrow = c(2,2))

for(i in 1:2){
   v1 <- sample(1:1000,50)
   v2 <- sample(1:1000,50)
   mat <- cbind(v1,v2)

   plot(v1,v2)
   boxplot(mat)

}
dev.off()

#######

The plot ordering is that the first row gets filled in first, then the
second row etc. But how can I change it so that in the code above, I get
both the plots in the first row and the both the boxplots in the second row?

many thanks!

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