I have been taking prednisolone for four years for polymyalgia. Recently I have been getting sharp pains back, what do I do?

Consider nutrition. First, go to your physician and get checked out. Since Prednisone is an anti-inflammatory med, see if you can take charge and start to heal yourself by eating an anti-inflammatory diet (find online). If you are feeling better, after a few months work with your doctor to see if you can slowly wean yourself off. Other anti-infl foods include tumeric, ginger, rosemary, apples, onions, etc.
Re chk the diagnosis. Pmr should not require 4 yrs on steroids! You and your doctor need to re visit other potential causes for your present symptoms.
Steroids & Back Pain. One concern with long term steroid use is osteopenia (soft bone) and osteoporosis. Bone fractures are not uncommon with these conditions. See your doctor to figure out the source of your back pain.