Originally Posted by mooshimanx
4 oz. is less than three shots of vodka and regardless of how cheap the vodka is, vodka by definition is flavorless, whereas bourbon and whiskies have a moderately strong flavor. Moreover, the original poster stated just above that he decanted the vodka before adding the nibs.
Someone else mentioned decanting their nibs and vodka, not OP. And yes, vodka is SUPPOSED to be a neutral spirit, but in reality that isn't always the case. Cheap vodka is frequently absolutely horrid with a strong, unpleasant taste. The fusels in poorly made vodkas in particular are definitely a sharp enough flavour to cut through a beer, even in small quantities. Again, there's a reason why when people make beers with whiskey/rum/whatever else the advice is always to use good stuff, not unpleasant swill. Cheaply made spirits with a strong fusel character can cause unpleasant off flavours.
And 4oz should be exactly 4 shots (sorry, I feel like nitpicking this), no? I know most shot glasses (at least in the bars I've worked in) are now made to be 7/8ths of an ounce because short pouring makes bars money, but that just works out to be ~4.5 shots. Maybe American shot glasses are larger than Canadian shot glasses?