If you have the longer springs (8 leafs usually, 56" overall length) you will NOT have to notch the crossmember. If you have the shorter light duty springs (52"), you will have to.
A word of advice from someone who has done a 4" rear drop...just do a flip kit and be done with it. It is HIGHLY likely you will want to go lower in the rear in the future, and with a 4" shackle/hanger kit...you really can't.
Welcome to the board!
