If the file is already sorted by date, you can do something like this

my $date = '';
my $count = 0;
while (<>) {
    @data = split(...);
    if ( $data[3] ne $date ) {
        if ($date) {
            print $date, "\t", $count, "\n";
        }
        $count = 0;
        $date  = $data[3];
    }
    $count++;
}
# Catch the last one
print $date, "\t", $count, "\n";
[download]

update: forgot last print