Had laminectomy & fusion l4-s1 25yr ago. not been able to work in 4 yrs due to severe pain- like a hole in middle of lower back- what could cause that?

Most likely there : May be a problem that has developed adjacent to your prior surgery over time due to the altered stresses of the fusion placed on that next disc segment which can lead to instability, disc herniation, degeneration and/or spinal stenosis. You need further evaluation by a spine surgeon which may include x-rays, MRI with contrast and possibly a ct.

L3-L4: Since you had a fusion the force of 4-5 and 5-1 has been transferred to the l3-4 level. Over the last 25 years it has slowly worn out and now there is deg disk dz as well as arthritis there. Sometimes pain mgmt can inject and help temporarily but it may be necessary to extend the fusion.