By using the command convert this is easily achieved.
This is the quickest way I've found to do this:
convert -page a3 infile.pdf -page a4 outfile.pdf
The outfile.pdf should now be sized as an A4 page.
The command is as follows: convert [input file options] infile.pdf [output file options] outfile.pdf
Of course it is possible to convert to and from other sizes as well.