3 is fine. Your not getting enough bang for the buck with 4 I find. 
does this relate to every power in the game. like resist in the invulnerable tanker and sheilds for bubblers
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I am guessing all AT are different, but is there any real point in going over the the 3 of the same type of enhancement in a power. Like i have a tank and I am wondering if slotting 4 damage resistance is worth it. If anyone can let me know.
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Different types of enhancements have differing values as SOs. For example, an Accuracy SO gives 33% Accuracy, but a Damage Resistance SO gives 20% Resist. Regardless though, after the value for a particular enhancement reaches the equivalent of 3 of that SO (99% or 60% respectively in the above examples) the value for another one drops off -dramatically-. A fourth Accuracy SO (or Damage or any of the other "Schedule A" 33% value enh.) will give you just 5% more boost. This is true for all ATs and all enhancement types.
With this kind of diminishing return, it is almost certain that your character could receive more benefit from that enhancement slot employed somewhere else. You cited the example of a Tank above. Remember that a Tank does not survive by its Resistances alone. You need Endurance Reducers to help pay for toggles that activate those Resistances. You need Heal enhancers in your self-heal(s) to refill your HP that are lost despite your Resistances. You need Recharge Reducers to make sure your click powers are up and available when needed. You need Accuracies and Damages and everything else. And you do not have all the enhancement slots you could want. Just maxing out a particular trait at the expense of other slotting will end up with you greatly weaker than another build that tries to wring maximum benefit from every single enhancement slot.
Is there any possible situation where a 4th SO would be warranted? Yes, but it would have to be something very specific. A tiny gap in coverage that the 4th SO would eliminate and give greater total benefit to the build than 5% or 3% of boost would provide in and of itself.
To your specific question above: A fourth damage resist provides approximately 3% more boost. That's three precent of the starting value of the power. So if you were, say, enhancing the Stone/ auto-power "Stone Skin" with a value of 12.7% Smash/Lethal resist, a fourth SO would raise the value from 19.5% to 19.88% (I'm assuming even level SOs).
A great aid to seeing what you are doing with your (potential) build is to use SherkSilver's Hero Builder. Some of the numbers in the powers are innaccurate, but in the main they are correct to the player-base's ability to determine and it's a tremendous tool.
cool thanks
I am guessing all AT are different, but is there any real point in going over the the 3 of the same type of enhancement in a power. Like i have a tank and I am wondering if slotting 4 damage resistance is worth it. If anyone can let me know.
Thanks