Thanks for all of the terrific ideas! I think I'll use them all!!!

Make the tree trunk by cutting up brown paper bags and wrinkling them up and shape it by stapling it to the bulletin board. Then cut out paper apples and staple them on tree branches. Write your children's names on the apples.

Once there was a little boy who came to visit his grandparents in a Norway. Well, the small child gets bored as their is not a lot of excitement in Norway. The boy asks his grandma what he can do. She said to go out in the town and find a round, red house that has no doors, no windows and has a star inside

The little boy ponders the idea but can not figure out what he is supposed to find. She decides to go ask farmer John from next door, as he seems fairly smart. She ask farmer Scott but he can't figure it out but he has an ideal.

He says, See that apple tree in my front yard, go sit under it and think lots of great ideas have come under an apple tree. So the little boy goes, sits under the tree and is about to fall asleep when out of no where a huge puff of wind comes. The tree began to sway and an apple fell out of the tree landing right next to the boy.
The boy picks up the apple looks at it, and suddenly it hits him; An apple is round, red, has not windows, no doors, but wait is there a star inside. He runs to his Grandpa and asks him to cut the apple in half and sure enough, there is a star insider the apple (at this point cut an apple in half and show the star)

You might try the archives of this board. Apples are certainly a teacher friendly theme. I saw vinyl table cloths and various other items at WalMart in their home section when I was there yesterday. If you use the Apple tree as suggested by the previous poster you can transform it into a winter tree, then Spring as you progress through the year.

How about for the outside bulletin board: "Falling into fourth grade". Use a bulletin board tree and apples falling from the tree (with kids names on them).

Other captions:

A bushel of....
An orchard of...
Don't be a bad apple! or "A bad apple spoils the bunch". (behavior management)
A great crop!
We're ripe for the picking!
Apple core, baltimore, who's your friend? (student name) is! (for the star student board).
etc....

Well, I didn't plan on theming this year but after I put up a bunch of stuff I saw that most of what I have used this year is "apples." Today I went to Dollar Tree and found TONS of stuff including apple soap dispensers so... now help me out with the slogans. This year will be my first in fourth (after several in fifth). Thanks, I know I can count on the creative minds here!