i think there's no exact japanese word for the translation of quadrilingual, but if i need to say that in japanese, i'd say like 四カ国語を話せる（よんかこくごをはなせる）. and "quadrilingual person" = 四カ国語話者　（よんかこくごわしゃ） btw i believe many japanese people know what bilingual means, so you can call someone who speaks two languages as バイリンガル.