Split an A3 in 4 A4

A trick for splitting an A3 in 4 A4

pdftk mapA3.pdf burst output pgmapa3%d.pdf
convert -density 600x600 -limit memory 32 pgmapa31.pdf pgmapa31.jpg
convert -verbose -crop 3508x4960 pgmapa31.jpg pgmapa31_%02d.jpg
convert -page A4 -resize '2480x3508^>' -extent 2620x3706 pgmapa31_0* pgmapa4.pdf
gs -sDEVICE=pdfwrite -dNOPAUSE ...
more ...