my $last;
my @history;
my $expect = 1;
while (<>) {
    chomp;

    my ($num) = /EVENT(\d+)/
        or next;  # Ignore bad input.

    next if defined($last) && $last eq $_;
    $last = $_;

    if ($num == 1 || $num != $expect) {
        @history = ();
        $expect = 1;
    }

    if ($num == $expect) {
        push @history, "$_\n";
        if ($expect++ == 4) {
            print(@history);
            @history = ();
            $expect = 1;
        }
    }
}