A lytic spondylolisthesis is less common at L4/5 than it is at L5/S1. It sounds as if he has associated foraminal stenosis where the pars remnant pulls the exiting root anteriorly and compresses it against the disc anteriorly, in combination with loss of foraminal height.
Conservative measures should always be employed first and nerve root injection may reduce symptoms. However, some patients get recurrence of their pain since the injection does not take away the underlying condition.
If surgical methods are to be employed, isolated decompression cannot be used for this situation since satisfactory decompression will destabilise the spinal segment.
My treatment of choice is often a posterior instrumented fusion with decompression and insertion of transforaminal lumbar interbody cage. I then resect the pars remnants and excise the superior articular process of L5 to achieve foraminal decompression.
Other options are anterior (or lateral) lumbar interbody fusion to restore disc and foraminal height, which achieves indirect decompression of the compressed roots.
Patients need to have all the information available to help them make the correct decision for them.