You can revert the files a few at a time. As a test, you could run p4 revert //path/to/some/file
and verify that it's able to revert that file.
Once you know that's working, you just need a way to automate the process.
You could script something up that starts in the root directory and runs through all directories breadth-first, running p4 revert //path/to/folder/*
at each folder (I think you could also use client paths).