THE POST OFFICE has issued several interesting statements regarding its new plan to add four digits to existing zip codes, but none has so caught our ears as that of Senior Assistant Postmaster General E. V. Dorsey. Mr. Dorsey, while defending the decision in terms of lower postal costs and rates, acknowledged that there are many who feel the additional numbers "increase impersonalization." That implies that the existing five-digit zip code is a relatively personal form of identification-something we had not heretofore considered. The question Mr. Dorsey poses is: Are we angry about the post office's plan because we will feel less pride as unique entities, less beloved, if you will, with four digits appended to our zip?

The logic of the plan, oddly, works the other way. The effect of the nine-digit number will be the other way. The effect of the nine-digit number will be to allow machines to quickly zero in on a much smaller area, such as a particular city block, in which we unique entities reside. Add a few numbers to the nine, and eventually the post offie would be able to zero in on one's very own house. (Naturally, that will engender cruel jokes asking why the post office couldn't use addresses instead of zip codes-but we'll let them pass.) The point is that sooner or later not only would everyone have a number on his house, but he would also have another number with the post office, which means that every time a letter came for, let us say, 200310393047, a government agency would be thinking solely of you.

Mr. Dorsey's question, thus, becomes something else: Is it the sheer length of a number that we resent, or is it the number itself?It cannot be the latter, surely, since we live rather comfortably with quite a few nimbers as is, such as phone numbers and Social Security numbers, which we have come to look upon as little friends. An American Express or Mastercharge number is no better than a fair-weather friend, of course, but that's because of the way it-ultimately-confronts us. Even a bank-account number, which runs a lot longer than nine digits, can be your friend, depending on the day of the month. So it must be the idea of extra, additional numbers that has people wary or irritated-a factor that has less to do with total length (our Social Security numbers have nine digits, too) than newness.

Then again there's the possibility, albeit remote, that we simply resent the number nine. There are nine muses, and no more; nine innings, and no more. A cat has nine lives. A stitch in time saves you-know-what. Perhaps it's the finality of the number that gets to us. Five, on the other hand, is a more human, a more day-to-day number, what with five fingers, five toes, a good five-cent cigar and a five-day work week-except for the post office.

Or maybe the matter is simpler still. Maybe we're just naturally suspicious of the world's most celebrated inefficient bureaucracy, which, for all its machines and zip codes, has yet to prove that it can deliver a post card to the right party at the right place on time.