My 4th vertebrae is slipping, i asked my doctor for a referral for mri he insisted physical therapy but last couple weeks my left leg goes numb? ideas

See a specialist: By your question I am assuming you have had a plain x-ray showing changes at your 4 th lumbar vertebret since you can't see disks on plain x-rays i suspect you have something called sponsylolysthesis of mild severity. Have you discussed the new numbness with your doctor. If so you may need to see a pm&r doctor or neurologist for a more comprehensive exam to see if more studies are needed.

Spine specialist eva: A listhesis is treated with therapy. Failure of therapy to help or progression of symptoms, like leg numbness warrants MRI evaluation with a spinal surgeon.