The umbilical cord should be long enough that it has no stress placed on it in sharp turns, but shouldn't have excess slack. It should be routed such that it won't be harmed by any pinch points. I like to tie my umbilical to the tow bar near the hinge point. That way the cable length is nearly the same at any angle.
Regarding safety cables/chains - the above regarding the umbilical cord applies to the length of the cables. Do the same with the cables as the umbilical - tie it to the towbar near the hinge point, and cross the cables - left on toad to right attachment point on the two vehicle, etc. Regarding the strength - you must meet or exceed the toad's weight, so 6K is perfect for your 4.5K toad.
The important part is the length of the break-away cable - it must be shorter than the safety cables - if the rig breaks away from the MH, then it must pull the break away switch before reaching the end of the cables.
The same applies whether towing a trailer or a vehicle.
Hope that helps!