For April 1st 2016 I migrated the main Raspberry Pi Wordpress blog on to 8 Pi 3Bs (not the + model). https://www.raspberrypi.org/blog/the-little-computer-that-could/
We were unable to migrate the (large) database backing the site onto the Pi 3 as it didn't have enough RAM to perform properly, the Raspberry Pi database is much larger than a typical Wordpress blog. In normal usage Wordpress Supercache was used to accelerate the Raspberry Pi blog so that the majority of pages are delivered as static HTML - around 90%. On a typical day the Raspberry Pi blog sees around half a million users which was adequately served from 8 Pi3Bs, our graphs suggest we'd have got away with just two.
For the launch of the Raspberry Pi 3, 5% of the page requests were served directly from a Pi3, approximately 1.2m. Launch day has a higher cache hit ratio than normal - vast numbers of people arrive, look at the launch page and leave so this is an over-estimate for a normal site. https://blog.mythic-beasts.com/2016/02/29/hosting-the-raspberry-pi-3-launch-on-a-raspberry-pi-3/
For a simple Wordpress blog, Wordpress Supercache installed with around a 75% hit ratio you should see page rendering in around 1s. You've four cores so you can render roughly four pages per second, and you get 4x that for free due to the caching. That works out at around 1.2m page requests in a day at perfect efficiency, allowing for a typical usage curve you'll get between 300k and 500k. If you want to try it for real Mythic Beasts run a Raspberry Pi cloud, https://www.mythic-beasts.com/order/rpi
Ben Nuttall runs PiWheels on this Raspberry Pi cloud, all of the downloads are done by a single Raspberry Pi 3B, which typically downloads over 20,000 packages per day.