Ok, so in general I would say that gifts that support the imagination, creativity, and the ability to run off all the energy would be winners in the long run. I'm guessing that you're looking for specifics...
Of course, it depends on the kiddo... I nanny a boy who has gone through 4 while I have been with him and he's a dinosaur nut. So anything to do with dinosaurs was a huge hit for him. So my suggestion is to just think about any big interest and support it in as many ways you can. From books to activities to outings. I think that's one philosophy behind "unschooling" is to follow the child's interests. And I think it's pretty great as a parent to foster that love of learning and have fun at the same time.
Imagination: we liked to use cardboard boxes to make all kinds of things... airplanes, trains, buses, etc. We would decorate them with crayons and markers. We also made a lot of forts, tents and general hide-outs with sheets and binder clips and furniture. These things we would play with for days. There is one thing I'm considering getting my son who also turns 4 in September, is the Cranium Superfort... anyways, check it out. We also did a LOT of dress up and costumes.
Creativity: honestly anytime we did a craft this boy has been extremely excited and happy. Even just letting him loose with markers or paint or crayons or glue was a big hit. So consider getting a bunch of art supplies or an art easel if you don't have one. Also, for yourself consider checking out books by Susan Striker... She's written one called the Anti-Coloring Book, and another one that I like called Young at Art. I think she's written others, and the one has reinforced in my mind the importance of my son being able to explore with art as an end to itself--the creativity being the goal... rather than creating something we can recognize.
Ok, back on track...
A close second with 4yo stuff would be musical instruments. They aren't something that they play with everyday, but when I remember to pull them out (harmonica, drum, accordion, maracas, etc) they go crazy for over an hour.
And I also know that anything that would let them run around, like a bike or outdoor activity is essential. I've considered getting the mini-fold up trampoline. They also sell a bouncy house at toys r us or other places that is around $100. My nieces and nephews have one and spend hours in it.
I forgot to add that the boy I nanny, well his parents read to him a lot. And as the stories have progressed, they have started reading chapter books to him... Like winnie the pooh, or stuart little. I've even thought about getting the magic treehouse for my son's birthday.
Oh, and I should add Board Games! There are lots of unique and different board games... Check out Walking Stick toys or Rosie Hippo or Magic Cabin... they have some intriguing games for younger kids. I know we just started playing hi ho cheerio (bought it at target), anyways, we also started playing dominos and go fish.
How could I forget Marble Ramps!!!!!! Days of fun right there