Talking to a 3 and 4 year old about death…
They were in my office when my dad called to tell me, and I said, “Oh no…” and Zander immediately asked what was wrong. Then I went to tell Doug, and of course, burst into tears. So the boys came in asking what was wrong. We looked at each other, and I said, “We should tell them.” It didn’t feel right to lie to them, so we each pulled a boy into our laps and sat on the bed.
Doug did all the talking. I don’t remember his exact words, but he basically said that Logan was very old and was not very healthy. And that he had died. And that “Mommy is very sad because she is going to miss him very much.”
Zevan (just turned 3) seemed more concerned about me being sad than about Logan dying. Later that day, I was feeling sad, and he was acting cranky, so I asked him if he would lay down with me for a few minutes. We cuddled on the bed and after about 20 minutes, he got up and said, “I feel better.” “Me, too,” I said.
Zander (4 1/2) has grieved for Logan, and the two times we’ve mentioned Logan since, his little eyes well up with tears and he starts rubbing them. It’s so heartbreaking.
But we tell him that it’s okay for him to cry about it if he feels sad about it. And that he might feel sad for a while. Doug was particularly vocal about him letting out his feelings. 
Now, our boys watch a lot of Animal Planet/Discovery shows where death is matter-of-factly portrayed as a part of life. So, we’re pretty sure they get what death is.
Ironically, they’ve never seen the scene in Lion King where Simba’s dad dies or in Finding Nemo where Nemo’s mom and siblings die, or even in Land Before Time where Little Foot’s mom dies. We skip those parts right now. I think having it anthropomorphized that way makes it too emotional for them right now. I don’t want them asking about our deaths just yet.