From talking to folks over the years at various dog shows, rescue days and whatnot I have been told that Bloodhounds CAN have issues with aggression, and particularly with food aggression. We just don't think of them being aggressive in any way because the stereotypical Bloodhound, as pictured in movies and TV, is always a dog who lays around and has to be begged to move even the slightest little bit. When she showed aggression toward your three little dogs was it dinnertime or were there any types of food or treats involved?
As with any breed there are good breeders and bad breeders. Under what circumstances did you acquire your Bloodhound? Did you purchase her from a breeder or did she come from rescue? If she came from rescue, aggression issues COULD be the reason she ended up there in the first place. A good breed rescue, however, would have put her into all kinds of situations to determine her reaction to things so that they would be able to determine the ideal situation in which to place her. If it was a shelter situation, depending upon how overcrowded they were, they may or may not have been real careful about their evaluation techniques and were just happy to have found a home to place her in. If she came from a breeder situation, the breeder may or may not have been totally honest about her attitude depending on how badly they wanted to find a home for her.
If the aggression seems to be motivated by food, then it should be easy enough to avoid by simply feeding her apart from the other dogs and making sure that they are never put into a situation where there is food being offered when they are all together. I would also contact the vet to ask for a reference for a good canine behaviorist. I would consult with them and see what can be done and how difficult it would be to do it before deciding to send her on down the road. It might be something that can be corrected fairly easily or it might not, but at least you will be rehoming her based on a professional opinion so you won't end up second-guessing yourself.
Ghilly
This is going to be one of two potential problems ... either your bloodhound is trying to dominate your other dogs and challenging to become alpha amongst the pack ... or she is confusing your smaller dogs with the furry creatures she would instinctively chase and kill when hunting with humans. Of course, while the former would eventually resolve itself, depending on the nature of your other dogs and depending on how you (as the pack leader) behave and deal with the dominance ... however the latter scenario could prove fatal for one or more of your smaller dogs.
I can only suggest you adopt extreme supervision over perhaps a week or two, if this is possible, and see how the pack behaves towards her. If this cannot be achieved, due to other commitments or an innability to supervise 24/7, then sadly you should probably rehome your bloodhound to protect your other dogs.
It would help if you knew her 4yr history. Who she was previously placed with, whether they went hunting with her, whether she lived with other dogs, small or large, and how she used to behave amongst other dogs prior to you getting her.
I have had several lurcher/greyhounds and while some seem quite happy to be amongst cats and other small creatures, others have not been able to avoid their instinctive behaviour to chase after them, even when they have never been in situations where this behaviour has been encouraged by humans or taught by another dog. Tony