How do I print A4 sized PostScript documents on letter sized paper?

psnup -Pa4 -nup 1 filename.ps | lpr -Pprintername

for example:

psnup -Pa4 -nup 1 mathematics.ps | lpr -Pprocyon

You can also output the document to a new file.

psnup -Pa4 -nup 1 filename.ps filename.ps.out