The brown button on the top says I have 4 total posts awaiting review, yet when I click on it, I get the screen above, showing I have 0 tasks to review.
Not exactly. That link says that there are four total posts awaiting review. It doesn't take into account that you've already performed those four review actions. The number will go down to zero once enough people perform the same review actions to clear the items from the review queue, plus a bit for caching.
Review has always been one of the most trying aspects of performance on the Stack Exchange network, as I understand it, so it's strongly preferable to limit the amount of work that each and every page view does, by not always keeping that number exactly correct.
It's a smaller deal on bigger sites, where there may be sixty pending review items, so you wouldn't necessarily notice that it was off by just a few. But I agree that it definitely stands out on smaller ones sometimes.
Although that is a pretty small price to pay for having pages that load in about 10% of the time it takes to blink.