Well, since he didn't SHOOT the dog, it's a moot point. Frankly, I am likely to point a gun at a dog that I feel might become a threat to me simply because they are fast, small (smaller than a person), and a lunging animal is harder to hit than a person so I'd want to give myself the added half second or so to save myself the possibility of grave injury.
The issue of them being there in the first place is a question we cannot answer because we don't know why they will say they were there. Any evaluation of the 4th Amendment requires an evaluation of all the pertinent facts, and we don't have the other half so no reasonable evaluation can be made here. If you feel you have a claim against the agency, speak to some attorneys and see how much it will cost you to roll the dice. I strongly suspect that absent any real damages, the cost will far outweigh any potential award.