Nothing against the postal service here. Rather, I was trying to turn a small PDF document, four A4 pages, into a booklet that could fit into a single A4 sheet, printed on both sides. Nothing frightful here, it's a simple command chain. Yet, some commands in the chain switch from the A4 format to letter, and I happen not to like their initiative 🙂

If you need to make such a booklet from file.pdf to booklet.pdf, and want it to stay A4 throughout the process, you need to do this:

pdf2ps file.pdf - | psbook | psnup -2 -pa4 | ps2pdf -sPAPERSIZE=a4 - booklet.pdf

For some reason, psnup and ps2pdf think they should turn A4 into letter. To force A4 in psnup, you use the -p option. ps2pdf uses the same options as ghostscript (gs), hence the -sPAPERSIZE=a4 there.
Also notice the dash in pdf2ps file.pdf -: if you don't use the dash, ps2pdf will output to file.ps, the rest of the chain will starve, leaving you with a funny empty booklet.pdf document.
ps2pdf is also a bit picky, and you need to explicitly tell it that it should read from the standard input (the dash) and write to booklet.pdf.

Have fun!