Capper,
Cortisone or epidural injections can be used as a diagnostic tool. The pain generator source is more likely the disc issues, based on your MRI result, as opposed to arthritis or osteoporosis. When cortisone is injected its targeted at a specific disc level. If you then get relief, even of a temporary nature, you know exactly what the problem is. Neurosurgeons often work in tandem with physiatrists or pain management specialists who use injections as diagnostic tools at specific disc levels to see what the pain source is. Best guess based on MRI is L4 and that's probably where you get the injection. It also could be osteoporosis or osteoarthritis or rheumatoid arthritis but based on your MRI probably not, although until you see a specialist they cannot be ruled out. A bone density scan is the test for osteoporosis. The MRI would locate any degenerative issues, as I already mentioned, and none were indicated in your summary. MRIs comment on those things as well as arthritic changes which are or can be visualized on an MRI, if that's what is happening. So based on your summary no evidence to support that, but a specialist should view the films and you shouldn't rely on the radiologist report.
Who ordered and read your MRI? The whole point of an MRI is to get an interpretation from a specialist as to the pain generating source and then further testing as noted above can be done to target the source more specifically. It doesn't need to be a guessing game, it can be identified.