You need to be very careful when producing A4 PDFs because some packages will reset the paper size on you.  Thus you should define the page size AFTER importing all packages:

\documentclass[10pt,twocolumn,preprint,natbib,authoryear]{sigplanconf}
\bibpunct{[}{]}{,}{a}{}{;}


\usepackage{times}
\usepackage{subfigure}
\usepackage{wrapfig}
\usepackage{graphicx}
\usepackage{color}

% NOW force pdflatex to use A4 paper
\setlength{\pdfpagewidth}{210mm}
\setlength{\pdfpageheight}{297mm}