Not having built a 4, I'm guessing a little bit here but it looks to me that would be troublesome. The roll/crossbar is positioned directly above the inverted-V shape of the fuselage stiffeners, which would carry the weight of the inverted aircraft after a flip. If you move the rollbar aft of that point without carefully re-engineering the support structure, you are exposing yourself to a fuselage collapse in the event of a tipover, which would result in your head/neck trying (with predictably poor results) to hold up the aircraft instead of the rollbar doing it.
If you move it aft without redoing the structure, you're an experiment waiting on a tipover before you know the result. If you move it aft and redo the structure, you're a test pilot waiting on a tipover before you know the result.
Either way, caution is advised.