My dog is always acting depressed?

your dog got used to this other dog that went away. they were friends so she is depressed because her friend left. you cannot explain to a dog that it was just for a little while. wouldn't you feel bad if your friend left the country? you having a new dog for 4 days is nothing they need time to get used to each other. i got a new cat and had a dog that never got along with cats or other animals due to the kind of dog he is. it tooks weeks to get him used to her now they cuddle together. you need to take things slow and keep them separate. slowly bring them together outside or into a room for a few minutes it will happen.