I have definitely dealt with many arthritic rabbits at many different stages and you are correct that it is not uncommon at 4 years of age for a rabbit to have arthritis, my rabbit Mango was diagnosed at that age and as he aged it became very debilitating and also my rabbit Maxine had debilitating arthritis.
There is nothing you can do to stop the process but a glucosamine treatment can help in some ways and I believe the Joint Support may have that in it – will it help? Maybe, maybe not, anything that says “support” is not scientifically proven to help and as far as I know these Oxbow treatments don’t have any scientific support – this does have glucosamine though and that has some scientific support as a supplement – NOT a cure though.
The metacam is a non-steriod anti-inflammatory (a pain medication) and it will not help cure arthritis either – there is no cure – only supportive care for this particular disease. I think you should see how she does with this and without it – you may not be able to see any difference but if you do then I would use it as needed. I’ve tried metacam in conjunction with tramadol and with buprenex and I could really only hope it helped so I continued as I felt my rabbits needed it.
With Maxine, I did try a laser therapy and that seemed to work (for awhile) and I also did accunpuncture and I don’t know if it helped (it may have had I been able to continue – had problems with vets going on maternity leave or moving out of state who were certified with accupuncture so I can not know if it would have helped with the pain).
I wouldn’t close her in her pen either – let her decide if she wants to run around (it would be good for her) or not.
It may eventually get to a point where you cannot treat it and it becomes way to debilitating (but that’s not the case with all rabbits). I did end up eventually putting Maxine and Mango to sleep because it was so bad for them (they were probably around 7 or 8 at the time). Maxine was a very large rabbit and her one of her legs became atrophied and she was just not doing well at all. Mango eventually was very bad as well and could barely move – he also contracted a URI which is why I ended up putting him to sleep along with the very advanced arthritis.
So basically from my experience you can certainly try different options and she can live a long life with this disease. You may have to eventually change her litter box to encourage her to use it but eventually she may not be able to (but that is probably a long way off at the age of 4).