How can I rename a specific file out of a directory using a batch script?

How can I rename a specific file out of a directory using a batch script?

Example:

C:/Folder1/
Folder2
Folder3
Folder4

Now, I want to rename the folder Folder4 to Folder4_[insert anything here] (by simply adding the last part to the name)

However, how do I get the specific folder name into a variable (without just typing it in with a set /p)

How can I do that?