From the documentation site you linked, Q4OS doesn't run inside Windows. The Q4OS installer runs inside Windows, and it stores the root filesystem for Q4OS as a file inside an existing Windows drive partition. I'm not personally aware of other Linux OS's that do either of these things, nor do I particularly desire a Linux OS that does either of these things. I can see it being useful if you want to keep your Windows installation fully intact. I imagine disk I/O performance takes a small hit.