If you get hit with a mag 4 KB attack, and have -4 Mag KB protection, then nothing happens. At least, that's how it's supposed to work. You need to overcome the mag protection before someone is affected.
So, if somebody hiw you with a Mag 4.01 KB attack, you'd be knocked down.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
Thanks Aett. That supports what I finally found on the wiki "If the magnitude of a status effect exceeds the target's protection, the target will suffer the affects of the status effect." The "exceeds" word being key.
I think I'll give it a try and go taunt Hami to see what happens. 
Worst case scenario I'll respec out of the 3rd KB IO and give it to one of my other PC's.
The same rule applies to mezzes, too. While it's common knowledge that you need to apply 2 Holds to a boss in order for it to work, it's not because the boss needs mag 6 (each Hold being mag 3, or two mag 2, or a combination) in order to affect the boss, it's because mag 3 is equal to the boss' protection. This can be easily shows with Mesmerize, the single target sleep in Mind Control: Mesmerize is mag 3.5, and will sleep a boss in one shot.
http://www.fimfiction.net/story/36641/My-Little-Exalt
For the record... 3 KB IO's will stand up to the worst Hami has to offer. 
If a knockback and a KB Protection have equal magnitude... which one wins?
I assume you subtract KB Protection from KB and if the result is less than or equal to zero then you are unaffected, but I want to be sure.
If I want to tank for a Hami raid, would 3 KB IO's be enough? I understand it has mag 12 Knockdown's (and I've experienced it LOL) and 3 would add to 12 which is what brings up my question. I have 1 IO now which works 90% of the time for me and I always intended to add a 2nd but I think I could squeeze a 3rd in if it would work.