The full solution is below, with a short explanation for each group. To avoid spoilers, play the puzzle or open the hints page before reading the answer panels.
ATOM, BYTE, PIXEL, and SEED all fit the category "Tiny units". Smallness connects them. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Baking staples
SUGAR / YEAST / BUTTER / FLOUR
SUGAR, YEAST, BUTTER, and FLOUR all fit the category "Baking staples". A bakery keeps these close. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Kitchen verbs
DICE / FOLD / SEAR / WHISK
DICE, FOLD, SEAR, and WHISK all fit the category "Kitchen verbs". These are actions from a recipe. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Desk items
STAPLER / LAMP / NOTEBOOK / PENCIL
STAPLER, LAMP, NOTEBOOK, and PENCIL all fit the category "Desk items". They belong within arm's reach at work. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.