I am a c4-c6 incomplete quadraplegic I am able to walk but lately have been loossing my abilaty to walk without sever low back pain. What might it be?

See doc. There are different reasons for you to lose your ability to walk, it's hard to say without knowing your medical history and what meds you are on. An important one to rule out would be a syrinx, or cavitation that can form in the spine, which can cause symptoms to worsen. See a spinal cord injury specialist, or your neurosurgeon if you have one, soon.
Examination needed. You should see a physical medicine & rehabilitation doctor (physiatrist) for evaluation of this. There could be one of several reasons for your symptoms, and a proper evaluation and accurate diagnosis is particularly important for you to maintain your function. Pain and weakness ought to be differentiated and assessed carefully.