Congrats!
I hope you have a wonderful experience, 4th graders are great. Is there any way you can go in for a week or so before the other teacher leaves? It would be very helpful for you to see how she runs her classroom. I would caution you to go slowly. I am sure you have many great ideas and of course you are anxious to try different things, but you will have a much better experience if you try to do things the same way the previous teacher did, at least for the first couple of weeks. Then slowly, you can start trying different things, I might even involve the kids in making changes, talk to them about things you want to try, get their input. Having them on board will really help. Above all don't talk to other staff members about the previous teacher or how the previous teacher ran the classroom. Good luck, it was a long term position that led me to getting my first permanent position.