Yup I got this error too
I fixed it by adding the method declaration to outfitter 4.0 (there's an API change for outfitter 4).
function Outfitter_GetBagItemInfo(...) return Outfitter:GetBagItemInfo(...) end
added somewhere near the top of outfitter.lua (lots of similar lines if you look) fixes the problem.
Probably need to rewrite baggins_outfitter to use new API.
Yeah that's kinda the wrong way around.. but thanks to that I think I found the the fix.
Baggins_Outfitter.lua line 13
local pItemInfo = Outfitter:GetBagItemInfo(bag, slot);
Baggins_Outfitter.lua line 16
local vOutfits = Outfitter:GetOutfitsUsingItem(pItemInfo);
As you can see it's simply replacing the _ with a :