The most probable source of trouble is the backlight. It is essentially a fluorescent light inside the monitor, and once it's gone it's gone. Newer monitors sometimes have LED backlights, but 4-year-old won't.
The amount of its remaining useful life hugely depends on usage pattern, you realize that. Occasional home use is not the same as office use.
The backlight often fails gradually, e.g. it becomes dimmer and dimmer until it is unusable.
If you need a correct colors representation (work with graphics or video professionally) - I think you'd better get a new one.
Some monitor's menu show its actual use time, in hours. I don't know about this particular one. But if it does show, you can know how heavily this monitor was actually used.
See how it looks with brightness at 50%. If it looks much too dim to you, probably its backlight is not in perfect condition. But you'll need to put a second known ok monitor side by side to compare.