documentclass{article}
usepackage{tikz}

begin{document}
begin{tikzpicture}[y=-1cm]
    draw[step=1] (0,0) grid (4,4);
end{tikzpicture}
end{document}

begin{tikzpicture}[y=-1cm]
    draw[xstep=1,ystep=-1] (0,4) grid (4,0);
end{tikzpicture}