Or search: quartet, fourfold, multiple, quartette, quadruplicate
1 Syllable: S
cruel, dual, duel, fuel, jewel, jewelled, jewels, Newell
2 Syllables: SU
brutal, bugle, crucial, ducal, feudal, frugal, fruitful, futile, neutral, pupil, rueful, scruple, truthful, tumult, tuneful, useful, youthful
2 Syllables: US
renewal
3 Syllables: USU
alluvial, approval, communal, perusal, quadrupled, refusal, removal, tribunal, unfruitful, unusual
4 Syllables: UUSU
disapproval

Found 37 words across 5 groupings. Skipped 12 excess words of a similar stem.