I always assumed the number next to the review link was the number of items in the queues, ans its non-matching caused by the queue being emptied (by others) without updating the indicator.
But now it's clear that this is not the case. Even reloading the page shows it still at (4), but the queues are empty and there is not enough activity to imply that new items are appearing and disappearing too fast to see.
I can't find 4 of anything, or non-zero of anything. So what's it for? It can't be counting what I did do because it appears out of nowhere (that's what prompts me to work the queues) and generally goes down in value.