Sorry to be the bearer of bad news, but production-type layer hens often experience health issues around age 3 or 4, due to the strain the high production of eggs puts on their reproductive system and body overall. They are bred to lay like crazy for two years for the egg industry, and then be culled. Some people keep them after that age just fine, but it's common to start seeing issues.
Since she seems lethargic, I would check her vent for signs of being egg-bound. Even though it seems like she might not lay any more, she may still pass occasional eggs that are soft and crack (and probs get eaten by the flock). I have an older production breed hen like this and have had others in the past. Also check for feather loss or discharge around the vent, as well as lash eggs. I never found out exactly what killed my two Golden Comet production layers, but we believe one had salpingitis (she passed a lash egg), and the other may have had some sort of reproductive cancer. These are both common in the production layers. We ended up culling them to put them out of their misery because even after giving them extra care, their quality of life dropped so low and they were suffering.
Sorry, not trying to be negative at all! I hope your girl doesn't have any of these. I just wanted to put this out there though so you know some things to check for.
If all of that checks out, I would just work on making her as comfortable as you can. Maybe construct a simple freestanding roost that is lower to the ground for her. Maybe add some poultry vitamin/electrolyte powder to her water to see if she perks up. Make sure she is getting decent quality feed. Occasionally give yogurt to make sure her gut health is good. Best of luck.
