Round A4 up to a multiple of 4 total pages. I can't easily get the number of actual pages processed, so rely on telling the (rather hacky, and surprising - I have to divide to a separate counter to force an integer result) macro the number of pages used before the current numbering started.

\padpages{4}{8}
\pagecolor{havocpurple}
\thispagestyle{empty}
\mbox{}