What you describe does not sound like hydraulic redundancy. Only fatal wing or tailplane damage could take out all 4 hydraulic systems- wingtip damage could only take out 2 systems. Each side of the tailplane is only served by 3. Only 3 hydraulic systems go to the fin. It should not happen to the 400 series. Engine damage, even traumatic, should only have very limited damage potential to the total hydraulic systems. That is why I said rather than try and improve on that system, other areas of risk need to be examined- floor collapse, rear bulkhead etc.
It is the designer who is the chief weapon against such common cause failures. Later developments incorporate a lot of improvements. The 737 hydraulic system and the way it was modified from 100/200 series to all subsequent models shows this.