It is unlikely that arthritis in L4 & L5 can cause compression that would lead to leg weakness, in the absence of stenosis.
Why did your doctor order a spinal (lumbar) MRI ?
Could you try to stay away from carbs, sodas (including diet sodas) by following a strict low carb diet for a couple weeks and do daily "Magnesium Oil" transdermal treatments?
Just do a search for the latter, or pm for details
You may surprise yourself with the results!
This is not intended as a substitute for medical advice
Long-term pain in your back and legs can lead to long-term inactivity, which in turn weakens the muscles surrounding your joints. This can be a cause of leg weakness and instability.
Have you seen a rheumatologist for the pain in your joints?
Having arthritis in the L4-L5 is degenerative disk disease in the lower spine. Not only can it cause you localized back pain, but can also lead to other problems like disk herniation and sciatica. What happens with a disk herniation, it bulges out and can sometimes press against a nerve or the sciatic nerve. You would feel pain run down the leg and this is called Sciatica.
Not only can you have referred pain and pins and needles and weakness in your legs from the lower spine, but degeneration in the L4-L5 area can also lead to bowel and bladder problems like incontinence.
Although your legs feel weak, you still need to walk and move about. Your doctor could refer you to physiotherapy to help you with specific exercises to do to strengthen the muscles in your legs.
Your elbow pains are a separate issue to your legs. You may not have an autoimmune arthritic problem, but osteoarthritis. A blood test would confirm whether you have an autoimmune problem like rheumatoid arthritis.
I have osteoarthritis in most of my joints now, the stiffness is the worst. I do get referred pain that runs down both my legs, pins and needles and a feeling of weakness all over, not just my legs.