Congratulations on your position. I ABSOLUTELY LOVE the fourth grade!
I have really tried to stop buying things - especially posters. Instead of buying them, I go to my local teacher supply store and take my digital camera. I snap pictures of any posters I like or I cut them out of catalogs. I then use the die cut machine at school to make the words and I print out pictures from the computer to make the same poster. Since I use the school's construction paper to cut out the letters and the school's ink to print the pictures, they only cost me my time.
As for major purchases - I have bought literature sorters to act as mailboxes for the students to pass out papers that need to go home. But you can do the same thing if you have the students bring in an empty cereal box. Just cut off the top and hot glue them together and when you turn them on their sides and they work just them same. You can cover the side with construction paper so that they look a little neater (but I actually think they look neat without covering them)
There are also a lot of games you can have your kids do on the internet (if you want some of the sites let me know) so that you don't have to go out and buy them. I use these as centers.
The most I have spent my money on is books for the classroom and even then I use the used book store and garage sales.