I need advice

This morning there was an accident. My 4 year old Pitt mutt attacked my 10 year old shih tzu in bed early this morning. They’ve had clashes in the past but this morning the 4 year old mutt attacked him out of nowhere. He injured my shih tzu pretty badly that my shih tzu had to have emergency surgery. He lost an eye. What measures should I be taking so this doesn’t happen again? Currently the 4 year old mutt isn’t allowed on the bed. Any tips would help.