I agree that a 4 year old will likely be genuine. They will at least say what they think they are seeing, but I also agree with Sullivan about jumping the gun a little. Ask the kid some dog questions like suggested, and do the ride by, and you'll probably get an idea of what is going on. As far as questioning the parent, unless you have something unmistakingly obvious I would leave that alone, besides if they are, do you really think they are going to admit it? Unfortunately with age often comes deception to fit our agenda.
If it looks like a duck, and walks like a duck, its probably a duck. I hate to put labels on people because of their lifestyle, or what kind of home they live in or car they drive, but if the kid seems to confirm your suspicion with more questions, and the appearance of the setting in question has a fitting look to it, I would consider contacting some kind of animal rights group, but when I hear ANIMAL CONTROL, I think 5 day reprieve at a kill shelter.
Be open minded, and don't let your first thought put a spin on things you find in further investigation. Act as though its fresh news. Just like a 4 year old confusing and twisting things, we as adults sometimes get stuck on an idea, and no matter what we hear, we hear what already thought, or start putting reason to something said to suit our gut feeling. ( He said this, but he must of meant that)
Back to the judging people on appearances. Be very careful and use some reason here too. Remeber who are we to say how another person lives, dresses, or places their priorities in where they spend their money? Only calculate the "where there is smoke, there must be fire" factor if the child is saying, 1. "yes my daddy makes our dogs fight, 2. Daddy has friends over to watch our dogs fight, and sometimes they bring their dogs to fight too. Daddy has to patch up our dogs after they fight." Something solid.
Another approach, ONLY IF YOU FEEL THOROUGHLY CONVINCED something is happening that should not be after some more questions, and a look at the setting in question, do a background check. This may sound like a lot, but anyone can go to their local clerk of courts office and view anyones criminal file if they have one. I'm not saying that if someone has made a mistake in the past, and manged to have a run in with the law they are obviously guilty. I have been arreested for something stupid in college 14 years ago that I am not proud of, but if you go and see a file a mile long, well back to the Ducks. Not everyone with a criminal record fights their dogs, I surely don't, but I would have to guess there are more people with records engaging in such activity, than people who have nothing more than a couple speeding tickets. I know how that sounds, but when someone decides to break the law, their reputation is tarnished, and no one is to blame for that but them.
I know this may sound over the top, and may seem just like outright snooping or taking it too far. But if you do get to the point where you feel you have no choice but to contact some kind of authorities on this matter, you will know that you defintely did not just overreact to a 4 year olds comments, and that you had very good reason to be concerned to the point where you took action. If it were me I would just want to be as sure as I could before I started making accusations. Good Luck.