My DD is also 4 years old.
We just did a magnet theme and she really loved it. I used the What Makes A Magnet book from the Let's-Read-And-Find-Out series. I get them from the library, it's a great series. I also did the magnet activities in the Science Is Simple book. Another fantastic resource for this age.
We have done the vingear/baking soda volcano when we went to the beach.
I have another book by Learning Resources, the Human Body Flip Chart. I think it is designed for adults, but I love the layout and the concepts are very simple. There is a flip page for the circulatory, skeletal, muscular, digestive systems, etc.
We do a lot of activities learning about other countries. Lately, we have been using Usborne's Sticker Dolly Dressing Around the World and Around the World with Mouk to explore different countries. We cook recipes from countries that are in the book, listen to music, etc. We have the GeoPuzzle Europe continent puzzle, and a Melissa & Doug US floor puzzle, both of those come out of the closet frequently.
DK's First Nature Encyclopedia is another hit in our house. We check out books from the library all the time on different animals, insects, weather...
For a color experiment, buy a white flower (carnations and daisies work well), and add food coloring to the water in the vase. In 24 hours, the flower changes color. It's a neat lesson on how plants 'drink' water.
Maya's summer camp was themed Animal Olympics. I thought that was a really neat idea that you can do a lot with (fastest animals, strongest, ones that jump the highest).
DD went through a solar system phase a few months ago when it was a theme at her Montessori school. We borrowed my parents telescope and looked at the moon, visited a planetarium, and learned about the planets (which ones are gas planets, the hottest, the coldest, the order from the sun...)
DD LOVES science and nature....we do a lot around those two things. The Magic Treehouse books are also a huge hit. She learned a lot about the rainforest from one of the MT books and it's non-fiction companion book.