There's no distinct rule that can be applied. It is mentioned in the Boss Book that new pedals often get labelled -2 or -3 because the want the pedals to appear as 'new' and not be confused with any older pedals. -4 is not used because that's an unlucky number in Japan.
according the The Boss Book, the PQ-4 somehow slipped through the numbering system. it didn't sell well, so Boss really considers the number 4 to be bad luck now....