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.
RAPIDS, BANK, CURRENT, and DELTA all fit the category "River words". Fresh water flows through this set. 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
SCORE / KEY / SURF / DASH
SCORE, KEY, SURF, and DASH 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.
Category
Night sky
NEBULA / ORION / VENUS / METEOR
NEBULA, ORION, VENUS, and METEOR 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
Things with keys
HOTEL / MAP / LOCK / PIANO
HOTEL, MAP, LOCK, and PIANO all fit the category "Things with keys". Each one can be associated with keys. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.