What is a non-displaced subacute fracture through remnant of l4 spinous process.callus is developing?

Not worrisome: probably not a worry unless its below lamina and may start to compress the collection of nerve rootlets below it over time - sounds like you had a trauma in the past, there was a small fracture of the spinous process (which in and of itself might be painful for a few weeks but likely will heal and come to nothing) and now the healing is leading to some bone formation in the vicinity