"I teach in four schools."
Translation:Učím ve čtyřech školách.
October 6, 2017
Correct locative is only čtyřech. See https://en.wiktionary.org/wiki/%C4%8Dty%C5%99i#Declension
Also, the word order is not the best one, but with right intonation it is possible in certain situations so I will add this wording.