Hello! I think I can help you with your hound mix!
He’s still young and developing at 4 months, which is great! An older dog may be harder to work with, but this shouldn’t give you too many problems at all, as long as you’re patient. Let me break this down a bit:
-He’s showing possibly aggressive (defensive?) body language.
-You’re not sure what activity takes place during the day while you are gone.
-Do you know how their lives were prior to your adoption?
< div>I know you’ve heard this answer before, but I would use rewards and praise when other animals (or people) are around the crate, to reduce the guarding behavior. You want to teach your pup that it’s ok when other animals/people are around, they don’t mean to harm him and he’s safe. Be consistent with this, and eventually he will want you to be around. Incorporate praise as well as treats. Reward them both for being around each other. Turn it into a fun ‘game’ if you can.
I am going to go ahead and assume you have a pretty good amount of experience with socializing dogs, and the importance of socializing at a young age, since you’ve taken care of many. I’m also guessing by ‘kennel’ you mean a large crate and not a kennel type enclosure?
Though some may disagree with me, isolating your puppy for six hours a day is absolutely fine as long as you work on socialization activities and offer play/exercise when you are home. You’ll want to work up to this point to avoid any anxiety behaviors from developing (separation anxiety probably being the ‘worst’ case scenario). If you have very little time for training due to work, try to begin on a Friday and work throughout the weekend.
Keep in mind, ideally it would take more than 3 days to get your pup feeling comfortable with extended durations of isolation. At four months, he likely won’t be able to hold his bladder all day; try not to exceed six hours (should be your maximum at this point). The American Kennel Club offers several articles on crate training, only a click away- if you need extra sources.
If you aren’t sure how the larger dogs treat the smaller one while you are away, body language will usually help. Does the smaller one show any signs of submission or anxiety when the larger ones are around? Ears back? Tail tucked? Crouching? Shivering? Or is he completely happy and playful?
Once you have this guarding behavior taken care of, you shouldn’t need to isolate them. You can also consider setting up a small camera for observation purposes, if cost isn’t an obstacle.