Welcome. Your 4-remaining need to be in a coop at night. That coop needs to be secure and any breaches secured with hardware cloth, You also need a hardware cloth apron around the perimeter of the coop and the run. Also, predator eyes (they have solar ones), are good for use with fox situations.
Hi and welcome to BYC; glad you joined! I'm sorry for your losses; have you been able to confirm for certain that the predator picking off your chickens is a fox? Unfortunately, most predators will continue to visit the "buffet" until all chickens have sadly been taken, and many chicken predators can easily climb trees. (Great Horned Owls are fearsome chicken predators too.) You can learn of all wildlife species in your county by visiting the iNaturalist website. I hope you are very soon able provide a safe home for your chickens!