Update: Unfortunately, neither of these approaches handles leading four-blank groups properly. Oh, well... (Update: Also, it seems to me that at some point the OPed example data changed from having groups of ASCII 0x20 blanks to having tabs. Is this really so? Only yueli711 can know. :)

One way (assuming a "space" is a blank, i.e., ASCII 0x20):

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'
[download]

Update: A slight variation if you have Perl version 5.10+ and the  \K regex operator:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '1    2            2        5                4';
 $s =~ s{ [ ]{4} \K (?! \d) }{0}xmsg;
 print qq{'$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'
[download]