my @which;
$which[$_] = "array1" for @array1;
$which[$_] = "array2" for @array2;
$which[$_] = "array3" for @array3;

my @pairs = @array4;
while (my ($one,$two) = splice(@pairs,0,2)) {
   print "$one and $two are in $which[$one] and $which[$two]\n";
}