Sounds like a dead or dying Power Supply Unit (PSU).
How long has this been happening? Or might more simply mean that the inside of the case needs a good clean-out. After four years you would be surprised how much dust and fluff can accumulate, such build-up would cause PC to overheat this in turn could be the cause of your problem.
Give the inside a good clean-out especially around all Fans and Air-vents and grills.