my %hash = ();

foreach my $file (qw/File1 File2 File3 File4/) {
    my %unique_in_file = ();

    open(IN, '<', $file);

    while (<IN>) {
        chomp;
        $hash{$_}++ unless $unique_in_file{$_};
        $unique_in_file{$_} = 1;
    }

    close(IN);
}

use Data::Dumper;
print Dumper(grep { $hash{$_} == 4 } keys %hash);