Using a 172k-entry word list, I get the following.
All unique letter sets that make more than 20 words:
(25) A, AE, AS, AT, ATE, ATES, EAST, EAT, EATS, ES, ET, ETA, ETAS, SAE, SAT, SATE, SEA, SEAT, SET, SETA, TA, TAE, TAS, TEA, TEAS
(24) A, AE, AL, ALE, ALT, AT, ATE, EAT, EL, ET, ETA, LA, LAT, LATE, LEA, LET, TA, TAE, TAEL, TALE, TEA, TEAL, TEL, TELA
(24) A, AE, AH, AT, ATE, EAT, EATH, EH, ET, ETA, ETH, HA, HAE, HAET, HAT, HATE, HE, HEAT, HET, TA, TAE, TEA, THAE, THE
(24) A, AE, AR, ARE, ARES, ARS, ARSE, AS, EAR, EARS, ER, ERA, ERAS, ERS, ES, RAS, RASE, RE, RES, SAE, SEA, SEAR, SER, SERA
(25) A, AE, AS, AT, ATE, ATES, EAST, EAT, EATS, ES, ET, ETA, ETAS, SAE, SAT, SATE, SEA, SEAT, SET, SETA, TA, TAE, TAS, TEA, TEAS
(24) A, AE, AL, ALE, ALT, AT, ATE, EAT, EL, ET, ETA, LA, LAT, LATE, LEA, LET, TA, TAE, TAEL, TALE, TEA, TEAL, TEL, TELA
(24) A, AE, AH, AT, ATE, EAT, EATH, EH, ET, ETA, ETH, HA, HAE, HAET, HAT, HATE, HE, HEAT, HET, TA, TAE, TEA, THAE, THE
(24) A, AE, AR, ARE, ARES, ARS, ARSE, AS, EAR, EARS, ER, ERA, ERAS, ERS, ES, RAS, RASE, RE, RES, SAE, SEA, SEAR, SER, SERA
(23) A, AE, AR, ARE, ART, AT, ATE, EAR, EAT, ER, ERA, ET, ETA, RAT, RATE, RE, RET, TA, TAE, TAR, TARE, TEA, TEAR
(23) A, AE, AN, ANE, ANT, ANTE, AT, ATE, EAT, EN, ET, ETA, ETNA, NA, NAE, NE, NEAT, NET, TA, TAE, TAN, TEA, TEN
(23) A, AE, AM, AT, ATE, EAT, EM, ET, ETA, MA, MAE, MAT, MATE, ME, MEAT, MET, META, TA, TAE, TAM, TAME, TEA, TEAM
(23) EN, ENS, EON, EONS, ES, NE, NO, NOES, NOS, NOSE, O, OE, OES, ON, ONE, ONES, ONS, OS, OSE, SEN, SO, SON, SONE
(22) A, AE, APE, APT, AT, ATE, EAT, ET, ETA, PA, PAT, PATE, PE, PEA, PEAT, PET, TA, TAE, TAP, TAPE, TEA, TEPA
(21) A, AR, ART, AT, O, OAR, OAT, OR, ORA, ORT, RAT, RATO, ROT, ROTA, TA, TAO, TAR, TARO, TO, TOR, TORA
(21) ER, EROS, ERS, ES, O, OE, OES, OR, ORE, ORES, ORS, OS, OSE, RE, RES, ROE, ROES, ROSE, SER, SO, SORE
(21) A, AE, AL, ALE, ALES, ALS, AS, EL, ELS, ES, LA, LAS, LASE, LEA, LEAS, SAE, SAL, SALE, SEA, SEAL, SEL
(21) A, AE, AERY, AR, ARE, AY, AYE, EAR, ER, ERA, EYRA, RAY, RE, RYA, RYE, YA, YAR, YARE, YE, YEA, YEAR
(21) A, AE, APE, APER, AR, ARE, EAR, ER, ERA, PA, PAR, PARE, PE, PEA, PEAR, PER, RAP, RAPE, RE, REAP, REP
(21) A, AE, AM, AMEN, AN, ANE, EM, EN, MA, MAE, MAN, MANE, ME, MEAN, MEN, NA, NAE, NAM, NAME, NE, NEMA
Of the 14,950 4-letter sets possible, 5239 make no words, 1271 make only a single word, 974 make two words, …, and 1 set makes 25 words. The full table of making is here:
1 25
3 24
4 23
1 22
6 21
7 20
19 19
27 18
40 17
68 16
89 15
131 14
174 13
240 12
328 11
370 10
524 9
689 8
792 7
920 6
1020 5
1071 4
942 3
974 2
1271 1
5239 0