It's impossible to say, since you've given us no information at all about either your PC, it's OS and drivers, or the games/programs you've been running.
Please post your PC's specs, which version of Windows you're using, the games you've been playing, etc. And I take it you've run a malware and a virus checker (if not, I recommend Malwarebytes Anti-Malware, and Avast!, both free), and found no infections? Also, download Ccleaner, and use it to delete your temporary files. I really doubt that it will solve your problem, but it can't hurt.
Also, you say you were getting a constant 60FPS, but are now getting 4FPS, how do you know? Are you running a program to check the frame rate?
Remember, no one can diagnose a problem without information. You're far better off giving too much information than not enough. --- "PCs don't catch viruses or malware. Stupid users do." - The Cranky Hermit. For all things N64: http://z9.invisionfree.com/Nintendo_64_Forever/
I play LoL also and it seems like it has been happening to some people since the latest patch with the graphics update. --- Kick in tha door wavin' the 44! All yah heard was poppa dont hit me no more! D;