I have a four-year-old girl!
Our all-time favorite author is Kevin Henkes. Some of his books are Chester's Way, Wemberly Worried, Owen, A Weekend With Wendell, Jessica, and Chrysanthemum.
My daughter loves anything small she can give a personality to and use to act out stories. She plays a lot with dollhouse dolls, and not necessarily in the dollhouse. She also loves to draw, so I'll add my vote for art supplies. And dress-up clothes are a huge hit, especially for playdates - we have a whole trunkful of stuff, from firefighter hats to princess costumes.
Games are great at this age, too. We have the usual Candy Land, Chutes and Ladders, Hi-Ho Cherry-O, etc., but also some less common ones like Sequence for Kids and some German made games that can be found in "natural toy" catalogs like Magic Cabin.
And last suggestion - outdoor play stuff - balls, sidewalk chalk, bubbles, hula hoops, gardening tools, bug collecting kits...