I recommend Usborne's See Inside books: How Things Work

For kids who can't yet read, pictures are everything. And to make that even more interesting, there are picture books with flaps. Usborne does some very good ones.

There's one in their See Inside series called How Things Work.

It is packed with explanations on how musical instruments or household appliances (hair dryer, toilet) work. It introduces new vocabulary (also perfect for me because I never really learned what parts of a crane or helicopter are called!). It's probably too complicated for four-year olds to really understand but there's no harm in introducing such topics. This will be a useful reference book for years to come.

These books are educational but so well done that they definitely belong in a kid's library. They would make great gifts, as well.