"Error in plot.new() : figure margins too large"

In R, I met a running error as follows:

> png("p3_sa_para.png", 4, 2)
> par(mfrow=c(1,2))
> plot(c(1:10), ylab="Beta",xlab="Iteration")
Error in plot.new() : figure margins too large
> plot(c(1:10), ylab="Gamma",xlab="Iteration")
Error in plot.new() : figure margins too large
> dev.off()
X11cairo 
       2 

I have already made the image size small to be 4 by 2, why it still complains "figure margins too large"? How can I solve this problem with png?

It is strange that if I change png to pdf, then it will work. I also wonder why?

Thanks and regards!