Com shells POSIX:
var1=col1,col2,col3,col4
IFS=,
set -f
set -- $var1
unset var2 var3
while [ "$#" -ge 2 ]; do
var2=$var2${var2+,}$1
var3=$var3${var3+,}$2
shift 2
done
com zsh
:
unset var2 var3
for a b ("${(@s:,:)var1}") var2+=${var2+,}$a var3+=${var3+,}$b