Explaining death (of dog) to 4 yo

[FromThe80s]

My mil's dog died recently - DD wasn't particularly fond of him but obviously noticed he's no longer there.

My husband told her the dog had died and was now in heaven. I didn't really want to tell her this as we're atheist and don't believe that - but she 'knows' about Father Christmas and the tooth fairy so I figured there was no harm

However, she's now asking more questions like where is heaven, how did the dog get there ("did nanny drive him there?") and I'm not sure what to tell her.

I'd rather be upfront with her but interested to know how others have explained death to their children - I don't want to scare her and understand how the concept of heaven is very comforting...

Could you frame the chat about heaven like "some people believe... No one knows exactly what happens..."
If there are questions regarding the actual death, I tend to say that dogs don't live as long as people do, or as long as we want them to, but dogs don't think about yesterday or tomorrow, they just enjoy today and don't mind that their lives are shorter.
I hope things are OK.

My dog died at Easter my 3 year old asked lots of questions so I just answered what I could. So he was very old and poorly and very sadly he died. I explained that it was ok to feel sad and that I miss him a lot. The more factual it is the better I think. I said we can't see him anymore but we can talk about him etc.