Reddit mentions of Ida, Always

As a childrens librarian I recommend The Heart and the Bottle and Ida Always. These usually get good feedback from parents. Both are aimed at teaching children to feel their feelings, and not bottle grief.

You might want to check out the Mourner's Bill of Rights and remember that both you and Sadie have the right to grieve as you need to.

Lifetimes is indeed a lovely book; I would also recommend Always and Forever, Are You Sad Little Bear?, and Ida Always, all stories about remembering a loved one that would be appropriate for a 4-year-old. There's also Chester Racoon and the Acorn Full of Memories (featuring the same characters from the story The Kissing Hand), in which Chester's friend dies and his class finds ways to commemorate him. Lastly, I Miss You is more informative rather than a story, but might help answer any questions she has.

I'm so sorry for your loss.

When my mum was sick earlier this year, I started reading my 4 year old books about death and loss.
https://www.amazon.com/Memory-Tree-Britta-Teckentrup/dp/1408326345/
https://www.amazon.com/Ida-Always-Caron-Levis/dp/1481426400
These first 2 are beautiful stories about losing someone but keeping their memory alive.
https://www.amazon.com/Something-Very-Sad-Happened-Understanding/dp/1433822660
And this one is more direct and helped my son understand what was going on after she passed away.

Kids are surprisingly resilient and if you're open and honest with her she'll be ok.