The industry standard is not set by the lift companies. It is set by the consumer. Most (not all) want their vehicles to sit level when they are lifted. There are options (add-ons) available that will allow someone to keep the stock rake when they add a suspension lift to their vehicle.
Some companies use the same brackets for the 4" and 6" kits. You could either put new leaf springs on the front or a 2" add-a-leaf. If you didn't get a new pitman arm with your 4" kit you will need one when you raise the front another two inches. As for the brake lines you can run your stock...