c:\@Work\Perl>perl -wMstrict -le
"my $s = '1    2            2        5                4';
 $s =~ s{ ([ ]{4}) (?! \d) }{${1}0}xmsg;
 print qq{'$s'};
 ;;
 my $t = '1    2    0    0    2    0    5    0    0    0    4';
 die 'bad transformation' unless $s eq $t;
"
'1    2    0    0    2    0    5    0    0    0    4'