Generally speaking, you want more then 4 pts. Especially if your a melee toon. You can work with 4 its just not a lot depending on your enemies. Of course if you get another Karma or some other KB protection (Blessing of the Zephyr, Steadfast Protection). Then I would say Acrobatics is not really needed.
If you PvP then you def want acrobatics and some more KB protection. I have no real data to give you here. I am sure a PvP'er will have better info.
KB is one of the biggest holes in Dark Armor. Having roughly 8-12 points of KB protection should get you through just about anything PvE. Anything over 12 is really overkill (IMO).
Again YMMV and you could be fine with Karma & Acrobatics. But if you want to save that power selection, then get yourself 4 more points.
It provides roughly half the protection of acrbobatics in magnitude terms
For PvE it will cover about 95% of knockback you come across, Cabal witches, Fake Nems are a couple of mob types that will still be able to knock you back.
Stacking with a second karma or a steadfast will up that to about 99%.
For squishies I tend to run with a single -KB IO, for a scrapper I like having the 2nd.
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Thanks guys, that was exactly what I needed to know.
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will that one IO provide enough protection from KB that I can now consider not taking Acrobatics, or would it behoove me to go ahead and take it anyway?
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It depends on how freaked out you'll get about being knocked back on rare occasions. I have a fire/fire brute who gets plenty of aggro and bought two of the Steadfast -knockback IO's (same magnitude as the Karma IO). I slotted one to see if that was enough. It reduced how often I was knocked back so dramatically that I sold the second one.
If you were a tank, I'd say you need at least two. Getting knocked back at a bad time could wipe the team. For a scrapper, do what I did: try it with one for a while. You can add another any time you feel like it.
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With one IO you will get knocked down fairly infrequently in PvE. Some mobs are more knockback-happy (Nemesis comes to mind), but one will keep you on your feet against shotguns and Council grenades. Think about slotting more if a) you PvP, b) you cannot stand being knocked back, or c) you are a melee toon who may often be at the epicenter of a large number of simultaneous attacks (one grenade won't knock you, but five at once will).
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I have a dark/dark Brute. With 0 -KB status effects in my powerset, I've found that Two -KB IOs fit perfectly into my build. If gives just almost 100% knockback protection from the majority of normal gaming and adds a bit of flair for AVs that have Knockback into their attacks. I still get knocked back from time to time by AVs with powerful knockback, but that is just for pleasure. I'd rather not waste a slot on an pointless IO to avoid that ever happening. 
In PVP on the other hand... I have 3 in my Corruptor, 2 in my stalker.
Once you get interested in PvP Recipes (If you do or don't, it won't be too much trouble if your don't,) you will find that some of them give -KB in the bonuses. I'm not sure on which and if it is only PvE or PvE/PvP. Might be something to look into @ Wentworths.
So I'm leveling a new scrapper (claws/dark) and just got a very, very lucky drop on a scanner mission: a Karma KB protection IO.
I've been gone from the game for a long while and I'm not that familiar with IO's, so....my question is this: will that one IO provide enough protection from KB that I can now consider not taking Acrobatics, or would it behoove me to go ahead and take it anyway?