Red Hat

$ head -3 /etc/inittab > firstpart
$ tail -6 /etc/inittab > lastpart
$ cat firstpart lastpart > /etc/inittab

$ cat > mynew4 <<eot
your new line # 4 here
eot
$ cat firstpart mynew4 lastpart > /etc/inittab

#!/usr/bin/perl 
# or wherever your perl is

use File::Copy;
my $orig = "yourfile";
my $backup = "yourfile.bak";

copy($orig, $backup);

open(IN, "< $backup");
open OUT, "> $orig");

my $newsetting = "your new line";

while ($line = <IN>)
    {
        if($line =~ m#"$newsetting)
            {
                $newline = "$newsetting";
                print OUT "$newline\n";
            }
        else
            {
                print OUT "$line";
            }
    }
close(OUT);
close(IN);