I would recommend Thornton Burgess’ books. Not the Animal Book and the bird Book at that age but just the stories like The tale of Little Joe Otter etc. There are a ton of them. Also the series that has the Meadow People, The Forest People etc. I think it is by Claire someone?? Sorry I can’t remember exactly. I can try and find it for you if you like.
Are You a Butterfly? (Judy Allen/Tudor Humphries) This is a series there is also Ant, Bee, Dragonfly, Grasshopper, Ladybug, Snail, Spider . My 4 year old LOVES these! The are shorter and give quite a bit of info about each insect.
Monarch and Milkweed (Helen Frost/Leonid Gore) Tracks the life cycle of the milkweed plant and monarch butterfly
Rain (Peter Speir) No words, but delightful illustrations!
Rabbits & Raindrops (Jim Arnosky) Jim Arnosky has ALL KINDS of great books, we just happen to have the rabbits & raindrops one. He’s a fabulous nature author/illustrator to look up by name.
Owl Moon (Jane Yolen/John Schoenherr) great horned owls
Not nature, but since you have a 4 year old, Over in the Meadow (illustrated by Ezra Jack Keats)
The Storm Book (Charlotte Zolotw) this one is also longer
not nature per say, but seasons The Year at Maple Hill Farm (Alice & Martin Provenson) follows farm animals thru the year. Also longer, but both my dc loved it.
The Among the People series is by Clara Dillingham Pierson and you should be able to find them free online. Have fun with Thorton Burgess!
Forgot one! It’s in the SCM science curriculum guide as a living book, also on FIAR lists: The Salamader Room, we just read it this week and it’s wonderful!!
I was just going to second the Thornton Burgess books. The chapters are nice and short so I think they a good for kids just getting used to listening to chapter books. My 6yo and 3.5yo love them. Many of these are available for free on Kindle too…