Knockback has always been kind of silly in Minecraft and the amount you take is heavily based on the consistency and latency in your connection to the server, but I can say for sure that 4 - 5 seems a bit ridiculous in my opinion. I'm pretty convinced that is not the case at all. Perhaps only if your knockback is so delayed that you can get hit by someone several times before you take knockback, adding it all up a few seconds later, thus set you flying to the moon.