Mere presence of a gap between L4 and L5 has no meaning. Perhaps you are referring to the X-ray finding. What looks like a gap on the X-ray is actually filled with tissue known as intervertebral disc. It is a protrusion of the disc that causes pain in the back radiating to the leg. You will have to be carefully evaluated by an orthopaedic surgeon before I can suggest any treatment as I have no details on physical findings.
Assuming your diagnosis is a disc prolapse there is no remedy for changing this ‘gap’. 80% of patients settle with conservative treatment with restricted activity followed by exercises. If you are not settling with conservative treatment only then would alternative measures be considered.