How can I remove the white lines in a colored multirow cell?

Question

I have a similar problem as in Remove white line in painted multirow tabular. Some PDF viewers like Adobe Reader and Evince display white lines inside my multirow cell.

thead/makecell doesn't work because I need to place the "Big Cell" at that exact position next to the three rows on the right. nicematrix sounds like a good solution, but unfortunately it's not available on my system (texlive2019).

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{multirow}
\usepackage{hhline}
\usepackage{tabularx}
\renewcommand{\arraystretch}{1.5}
\setlength\arrayrulewidth{1pt}
\newcommand{\cellgrey}[1]{\cellcolor{gray!40}{#1}}
\begin{document}
\renewcommand{\arraystretch}{2}
\newcolumntype{Y}{>{\centering\arraybackslash}X}
\begin{tabularx}{\textwidth}{ |*{5}{Y|} }
\hhline{|-|-|-|-|-|}

\cellgrey{} & \cellgrey{1} & \cellgrey{2} & \cellgrey{3} & \cellgrey{4}\\
\hhline{|*1{>{\arrayrulecolor{gray!40}}-}>{\arrayrulecolor{black}}|*4{-|}}

\cellgrey{} & \cellgrey{5} & \cellgrey{6} & \cellgrey{7} & \cellgrey{8}\\
\hhline{|*1{>{\arrayrulecolor{gray!40}}-}>{\arrayrulecolor{black}}|*4{-|}}

\multirow{-3}{*}{\cellgrey{Big Cell}} & \cellgrey{9} & \cellgrey{10} & \cellgrey{11} & \cellgrey{12}\\
\hhline{|-|-|-|-|-|}


\end{tabularx}
\renewcommand{\arraystretch}{1.2}
\end{document}

Answer 1

This white lines seems tO be pdf viewer artifact. I can reproduce them with ma recent MiKTeX and Sumatra pdf viewer. However, using tabularray package, they disappear (the same will happen if you will use nicematrix package for setting of your table):

\documentclass{article}
\usepackage{xcolor}
\usepackage{tabularray}
\begin{document}
\begin{tblr}{hlines=1pt, vlines=1pt,
             colspec = {*{5}{X[c]}},
             rowsep  = 5pt,
             rows    = {bg=gray!40}
             }
\SetCell[r=3]{c}    Big Cell
    & 1 & 2 & 3 & 4     \
    & 1 & 2 & 3 & 4     \
    & 1 & 2 & 3 & 4     \
\end{tblr}
\end{document}