Some kids are naturally very curious and he may simply have been curious about the "dangly bits" on the animal, but I too have not come across this type of behaviour before in young children.
When you caught him doing it, you could have taken the opportunity to ask why he was doing it and had he seen someone do this to an animal.
As none of the parents have any animals, then he won't be doing that at the parents' places (I presume that they live apart). As he only has access to the animals at your property and you have now separated the animals to not be anywhere near him, then leave things as they are and do not bring the subject up again.
If you do see the child doing something that is unacceptable and not right, do not shout at him. Just calmly take him away from the situation and explain to him that whatever he was doing was not nice. You can even say that you felt hurt by what he did if he is one of those children that asks "why" all the time and you get stuck for answers. Do reassure him that you still love him.
Do remember though that children have a VERY short memory span. Only put him right and tell him when you see him doing anything "naughty" at the time he is doing it and not hours or days afterwards.
As a grandmother myself, I understand your concerns.
Best of luck.
Since I caught him doing this to my cat, I haven't let him touch the cat and I try to keep him away from the cat. I did talk to his father about it, but all he did was yell at me for having a cat. I'm wondering if it is possible that he seen someone do the same thing to an animal or a person or something like that.
You should speak to the father, like AnnieBrooke mentions.
Speak to any adults who have him to make sure that they keep an eye on him to ensure that he does not does this.
It may be a good idea to keep the child away from the animals altogether.
Have you spoken to his father about it?
I explained to him that it's not nice to touch the private of others including animals. He doesn't watch these type of things at my house and I usually have him most of the time. Every time he comes back from being at his mother's house, he comes back doing something new that I feel is inappropriate and I can't talk to his mother about it because every time I try to talk to her about something he does or says, she says that he must have gotten it from my son's house (his father) or my house. I don't know what to do, I feel like I have to watch him closely with my 2 almost 3 year old granddaughter out of fear that he may try to do the same thing to her.
I would not allow this to happen and would say that this is not normal behaviour for a young child to be doing this.
Explain to the 4 year old that it is not right to this to any animal. Also explain to him the importance of washing his hands thoroughly after touch animals, because of any bacteria, parasites and other nasty germs.
Has been watching any vet programmes that have been examining animals?
You will need to speak to his parents to let them know that this is happening (may be they already know, may be not).