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.
SUSPECT, EVIDENCE, MOTIVE, and ALIBI all fit the category "Mystery story clues". A detective asks about these. The shared link accounts for all four words, which is why the remaining board words should be tested against a different rule.
Category
Tiny units
BYTE / ATOM / PIXEL / SEED
BYTE, ATOM, 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
Things with bars
MUSIC / SOAP / SIGNAL / CHOCOLATE
MUSIC, SOAP, SIGNAL, and CHOCOLATE all fit the category "Things with bars". Bars can describe 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
Things with lanes
POOL / BOWLING / ROAD / TRACK
POOL, BOWLING, ROAD, and TRACK 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.