What are the health risks when vit D is only 4.9ng in the body? Can it cause joint swelling, acne outburst and weight gain? What's the treatment?

Low Vit D. A level of 4 ng is very low. Would go for supplementation consider and discuss with MD 1000-2000 mg Vit D a day. See if your symptoms resolve. For the joint swelling would follow up this week with doctor /urgent care to assess if it is autoimmune or other pathology.