Hi Clare,
Thank you for that information, 4GB of RAM is the bare minimum for Windows 11, the operating system alone will use 2.5GB of your RAM, then if you have integrated graphics, that will also use more shared RAM, so that leaves maybe 1GB of RAM for your running applications.
So for example if you had a browser open with a few tabs, you would be using up most of your RAM, then Windows starts to use Virtual Memory (PageFile) on your drive, because it has run out of RAM.
You can continue using your PC like that, but it is not good for Windows to be constantly accessing your drive as virtual memory, really the best option would be to install another 4GB of RAM to bring the total to 8GB of RAM.