I get the full +4 (+2 plus +2) when saving versus fear.
+2 save vs fear
+2 save vs mind-affecting
This isn't exactly correct. You get it against *some* fear (notably the spell kind). NWN's default modifiers are very badly adjudicated vs pen and paper D&D. Many effects are not properly labeled as can be seen playing with modifiers or a Slippery Mind feat. Default auras for creatures are missing mind-affecting labels so modifiers aren't affecting their saves and Slippery Mind doesn't fire (dragon/creature fear auras/abilities notably ignore it) and many spells are missing proper tags (+vs mind-affecting won't work vs hold spells).
In short, the belt won't work the way you think it should (the right way) vs most encounters it should come into play, giving you only +2 instead of the proper +4.
This is usually the main reason why Slippery Mind is deemed a waste of a feat as many an annoyed rogue has cussed it out as it doesn't fire vs that dragon aura or wizard's hold person spell (although hold X might have been fixed in 1.69, too lazy to look).
