If you've broken 4 Exo's, you probably should look into your technique.
I've never had a problem with them, but I'm not a crazy hard hitter. But I've never seen a pro break them and you figure the Bryan Bros are taking some pretty heavy pace with their EXO3s.
Right now: Yonex Ezone Xi 100 w/ VS gut 17
In the bag: Hybrid Hornet MP w/ .5 oz tape, Babolat APDGT, Prince TT, Prostaff 6.0.