That would be something along the lines of:

my $prev;
my $data;

while ( $data = $sth->fetchrow_arrayref )  
{
    conditional_print ($data, $prev);
    $prev = $data;
}

#repeat for last record
conditional_print ($data, $prev);

$sth->finish;
$dbh->disconnect;

sub conditional_print
{
    my $data = shift;
    my $prev = shift;
    
    if 
    ( 
        $data->[0] eq $prev->[0] && 
        $data->[1] eq $prev->[1] && 
        $data->[2] eq $prev->[2] 
    )
    {
        print $data->[4], "\n";
    }
    else
    {
        print join ("\t", @{$data}), "\n";
    }
}
[download]

Basically you just have to remember the previous record somewhere.