Will severe compromise in the spinal canal L3 & 4 and l45 cause numbness in legs and buttucks?

Yes. Sounds like you have multi-level disease and perhaps a functional or anatomic spinal stenosis. You may experience symptoms when doing exertional activities, and the process can gradually and steadily progress due to wear and tear. The numbness is an initial problem, but further issues could include pain, weakness, urinary inability. Contact a spinal surgeon and see what can be done.