Answer: The correct reporting depends on how the physician documented his original plan of care, and upon the payer.
For Medicare, the physician should only bill for the successful procedure -- 64483 (Injection[s], anesthetic agent and/or steroid, transforaminal epidural, with imaging guidance [fluoroscopy or CT]; lumbar or sacral, single level) once for the L5 selective nerve root block, regardless of the number of injections he planned. You might be compliant in appending modifier 52 (Reduced services), but it depends somewhat on the documentation.
Take note: Your coding will change if the provider had not planned both level injections. If he first attempted L4 (which failed) and then successfully administered the injection at L5, you can only bill a single level injection code (64483).