Depends on how much it's been used over those four years, and how much current draw your rig uses. The batteries do need to be replaced after a while. Mine is about that old, and in the beginning, I was supposed to get up to 15 minutes of uptime. Now? I'm lucky to get 20 seconds; just barely enough to shut my PC down.
A UPS isn't supposed to be used to keep you running until the power comes back on (except for a few seconds), but to enable you to shut your system down safely to prevent data loss in the event of a prolonged power failure.
There is also the possibility that the surge suppressors inside the unit are useless; over time, repeated absorption of spikes on power lines slowly kill the varistors, eventually rendering them useless when you'd need them most!
Tom