This is something I've been wondering for a while. I've looked at a lot of pet IDs and I have not seen many 4mil. I've seen plenty of 3mil and 5mil, but until I found [pet not found] in the pound, I was beginning to think that maybe there weren't any 4mil.

What's odd is I have a pet born 10/4/15 with an ID of 3737223 -

Shuu Iwamine - and Lilliahna was born just a month and a day later on 11/5/15. I have a hard time believing there were 700k pets created between the two. (And there's

Idlewild who's born on 10/17/15 and has an ID of 3739031, just a couple thousand more than Hiding in two weeks.) Plus if you search the ID numbers just a little above and below Lilliahna's number, you find that these pets don't exist.

So what happened? The numbers just don't make any sense.

there was a HUGE pet purge that happened, a lot of pets were in the AC and they all got deleted, there was a second purge after but it wasn't as huge as the first. Subeta also cracked down on inappropriate pet names. I think it was around 2015 but I can't be positive.

There used to be a bug where whenever you refreshed to create a pet and got the 'this name is taken' error, it would take an ID. Combine that with the massive amount of Autorefreshing made by people to get names, you get a whole 1 million IDs gone in a short while

On top of what the others said, I believe there was a certain account clearing around the time that had a lot of attention from a lot of people. Sometimes glitches occur where they don't clear on the right day so a couple of days of a few dozen people refreshing caused the IDs to jump really quickly.

2mil IDs are more common than 4mil IDs but they're rare for the same reason.