They also omitted the need to show the license number (which I smeared-out in the older pic). I know that the parking cops have handheld devices, so I figure that they just look-up the registration # on the sticker. But that's a good question about renewing. Technically, they should be able to treat it like car registration, and have it come up as "expired" if you haven't paid for renewal, rather than having to issue new stickers every year.
OTOH, with 4 digits, they can only handle 10,000 people, which is probably adequate for one year, but wouldn't work if they kept getting new requests every year. There are less than 10k year-round residents, but about 25k in the summer.