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.
SEED, BYTE, ATOM, and PIXEL 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 / FLOUR / BUTTER / YEAST
SUGAR, FLOUR, BUTTER, and YEAST 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
Things with lanes
TRACK / POOL / BOWLING / ROAD
TRACK, POOL, BOWLING, and ROAD all fit the category "Things with lanes". Movement is organized into lanes. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Ocean words
KELP / REEF / WAVE / TIDE
KELP, REEF, WAVE, and TIDE all fit the category "Ocean words". They belong near salt water. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.