My 4th slot

What is +to hit worth compared to +Accuracy?

For exemple, is Kismet unique better than a 4th LotG in a defense power or is it just a 3% waste?

+6% to hit is worth more than 6% accuracy. Exactly how much more depends on the power in question, its slotting, your set bonuses, the level of the enemy you're fighting, etc etc, but as a ballpark figure you're probably looking at ~10-12% difference in final hit chance, for an IO build. It's possible to make up the difference elsewhere, though.

Whether that's better than an LotG depends entirely on the build and your preferences. I like to build for capped hit chance against +3s, which is rather more difficult without Kismet; Kismet is yet more important if you build for +4s, but not very important at all for Super Strength because Rage provides all the +tohit you'll plausibly need.

Because of the way the hit chance formula is written, if you have a lot of accuracy bonus, but not much, if any, to-hit bonus (this is the great majority of characters), to-hit is going to be a lot more valuable in increasing your actual hit chance. Conversely, if you have a lot of to-hit, but little accuracy (rare, Hopeling's example of a Super Strength user stacking Rage is one of the most prevalent of these, Time Manipulators using Farsight are another), the accuracy is probably worth more to you.

For more information, read this: http://wiki.cohtitan.com/wiki/Combat_mechanics.

I24 Proc Chance = (Enhanced Recharge + Activation Time) * (Current PPM * 1.25) / 60*(1 + .75*(.15*Radius - 0.011*Radius*(360-Arc)/30))
Single Target Radius = 0. AoE Non-Cone Arc = 360.

To-Hit is worth more.

To-Hit adds to your chance directly, while accuracy adds a percentage.

Okay, base to-hit for most powers is 75%. A 9% accuracy bonus adds 9% of 75 to your to-hit chance, which changes it to 81.75%. A 6% to-hit bonus increases it to 81% flat. So, that means the accuracy bonus is better, right?

Wrong.

What that +6% To-Hit does is more than simply adding to your to-hit, it is changing what the accuracy is adding percentages to. If you have a 6% to-hit bonus it makes your to-hit 81%. That 81% is now what the accuracy bonus is adding a percentage to, instead of 75%. That 9% accuracy bonus added 6.75% to your final to-hit. After you add the 6% to-hit, that exact same bonus is now adding 7.29%.

45% worth of accuracy bonuses add 33.75% to 75% for a total of 108.75%. If you have the 6% to-hit bonus it adds 36.45% to 81% for a final to-hit of 117.45%. If you add another 9% accuracy bonus it will add up to 115.5. With the Kismet, one more 9% bonus brings your final to-hit up to 124.74%

So, in short, you want BOTH slotted in your build, because the to-hit bonus of Kismet makes any and all accuracy bonuses you have from sets that much more valuable.