Exactly 4 Ss

5, 7-letter words with the letter -s- repeating 4 times:

assists possess sassies sissies swisses

Exactly 3 Is

26, 7-letter words with the letter -i- repeating 3 times:

bikinis finikin gingili idiotic ileitis illicit illitic incipit incivil indicia
inhibin inhibit initial insipid iridium irising lipidic lixivia militia minikin miniski rilievi ripieni silicic simioid vivific

Exactly 4 Us

1, 7-letter word with the letter -u- repeating 4 times:

muumuus

Exactly 4 Os

7, 7-letter words with the letter -o- repeating exactly 4 times:

booboos boohoos hoodoos hoopoos koodoos oloroso voodoos

Exactly 4 Es

11, 7-letter words with the letter -e- repeating exactly 4 times:

beebees electee exegete freebee peebeen peewees
referee teepees veepees weeweed weewees

Exactly 4 As

7-letter words with the letter -a- repeating 4 times:

atalaya