No, if you running Snow Leopard or Leopard. XCode 4 requires Lion. I remember that in some circumstances it's possible, but I really don't recall right now.
We (the students and me) didn't upgrade yet. They have older machines and for myself I also have to upgrade MAMP and other stuff and i'm not very inclined to do that.
Will look it up and post here if I find out.
Thanks.