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.
ACORN, EGG, TURTLE, and CLAM all fit the category "Things with shells". Each can have a protective outer layer. 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
FLOUR / SUGAR / BUTTER / YEAST
FLOUR, SUGAR, 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
Night sky
ORION / VENUS / METEOR / NEBULA
ORION, VENUS, METEOR, and NEBULA all fit the category "Night sky". Look up after sunset. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Words before board
DASH / KEY / SURF / SCORE
DASH, KEY, SURF, and SCORE all fit the category "Words before board". Put one word after each answer. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.