As PP said utilize this website it has a myriad of information from 4th grade teachers in this section, the busyboard and the archive.
I hope once you get in the school and room you find the prev teacher might have left some things

maybe they retired and left it all! I know you said you were called back so I don't know if that means it was an expected or unexpected vacancy.
As for math...I don't know what your state curriculum mandates but I find the more manipulatives the better. Especially with whole numbers, fractions and if you cover geometric shapes. That was the area as a student I was weakest and actually had a lot of trouble with math beginning in 4th grade. Again if anything was left in the classroom as far as manipulatives that will be so helpful for you getting started. Also finding out for your planning purposes when any statewide assessments are is critical too.
It is a lot of planning at first but hopefully you will find a great group of teachers on your new team to see you through. Congratulations and best of luck!